Add retain basic implementation on GNU/Linux

Retain is saved in file and backup of this file is done every time.
File isn't saved on every PLC cycle to avoid high I/O load.
By default, retain is saved every second and on PLC stop/shutdown.
It's possible to override retain flush period by using CFLAGS option
in project's settings. For example, for 0.5 seconds period
use "-DFILE_RETAIN_SAVE_PERIOD_S=0.5".

Add helper function GetRetainSize()

it will be useful for retain implementations.

use CLOCK_MONOTONIC instead of CLOCK_REALTIME for timer setup

CLOCK_REALTIME is a wall clock, that could change back and forth (user
changes system time, winter/summer time, NTP corrections and so on).
For real-time application CLOCK_MONOTONIC should be used, it doesn't
have such jumps.

Add function example to first_steps example

It'll be used in documentation to show how to create function in
Beremiz.

fix type introduced by f33942053466 (fix pylint warning "(undefined-loop-variable) Using possibly undefined loop variable 'X'")

Build by default with optimization level -O2 for GCC

before -O0 was used by default, that caused pretty bad performance.

Amd64, i6700k, 4200MHz, GNU/Linux (non-RT kernel), gcc 7.2.0

-------------------------------------
Optimization | EN/ENO |no EN/ENO |
-------------------------------------
default | 11 | 9.5 |
-O3 | 3.9 | 5.2 |
-O2 | 4 | 4.8 |
-Os | 4.1 | 3.5 |
-Ofast | 3.9 | 5.2 |
-------------------------------------

ARM, BBB Cortex-A8, 600Mhz, GNU/Linux, gcc 4.6.3

-------------------------------------
Optimization | EN/ENO |no EN/ENO |
-------------------------------------
default | 273 | 226 |
-O3 | 141.8 | 106.2 |
-O2 | 142 | 107 |
-Os | 152.5 | 112.2 |
-Ofast | 141.7 | 106.2 |
-------------------------------------

For embedded systems with size constaints (like Cortex-Mx, AVR and so
on) I usually use -Os. It gets pretty good results. For
GNU/Linux-based systems -O2 is usually a good choice, as you see the
test results.

fix hangs on mouse selection in case if wxPython uses wxWidgets with GTK3+ support

This is default configuration on Debian Buster+ and Ubuntu 18.04+.
Probably it'll fix completely problem with 'clipboard already open'
described in the previous commit 5ce6d08f ("make clipboard open minimal time as wxPython documentation recommends")

make clipboard open minimal time as wxPython documentation recommends

https://wxpython.org/Phoenix/docs/html/wx.Clipboard.html#wx.Clipboard.GetData

"Call wx.Clipboard.Open to get ownership of the clipboard. If this
operation returns True, you now own the clipboard. Call
wx.Clipboard.SetData to put data on the clipboard, or
wx.Clipboard.GetData to retrieve data from the clipboard. Call
wx.Clipboard.Close to close the clipboard and relinquish ownership.
You should keep the clipboard open only momentarily."

Maybe it makes situation with pretty annoying error 'clipboard already
open' a little bit better.

traceback:
File "/home/developer/WorkData/PLC/beremiz/avangard-beremiz-ide/src/../../beremiz/BeremizIDE.py", line 955, in OnSaveProjectMenu
self.RefreshAfterSave()
File "/home/developer/WorkData/PLC/beremiz/avangard-beremiz-ide/src/../../beremiz/BeremizIDE.py", line 946, in RefreshAfterSave
self._Refresh(TITLE, FILEMENU, EDITMENU, PAGETITLES)
File "/home/developer/WorkData/PLC/beremiz/avangard-beremiz-ide/src/../../beremiz/IDEFrame.py", line 926, in _Refresh
self.RefreshFunctions[element]()
File "/home/developer/WorkData/PLC/beremiz/avangard-beremiz-ide/src/../../beremiz/BeremizIDE.py", line 766, in RefreshEditMenu
IDEFrame.RefreshEditMenu(self)
File "/home/developer/WorkData/PLC/beremiz/avangard-beremiz-ide/src/../../beremiz/IDEFrame.py", line 1185, in RefreshEditMenu
if self.GetCopyBuffer() is not None:
File "/home/developer/WorkData/PLC/beremiz/avangard-beremiz-ide/src/../../beremiz/IDEFrame.py", line 956, in GetCopyBuffer
if wx.TheClipboard.Open():
File "/usr/lib/python2.7/dist-packages/wx-3.0-gtk3/wx/_misc.py", line 5793, in Open
return _misc_.Clipboard_Open(*args, **kwargs)
<class 'wx._core.PyAssertionError'>: C++ assertion "!m_open" failed at ../src/gtk/clipbrd.cpp(598) in Open(): clipboard already open

Merged GPL + LGPL v2 or later EtherCAT extension. Fixed headers and copyright notice.

Propagated matiec changes in FB var accessors

Fixed most obvious bugs appear after grafting changes from previous development branches : re-enable openning non-CoE, CoE and CiA 402 nodes config

Fixed xslt stylesheet for extracting entries defined in module, rewiting xslt stylesheet to yslt

Fixed bug in information returned for module where entries are defined only in PDO mapping

Added support for loading XML file even if not following XSD schema (but still following XML syntax), warning user of errors in XML file

Fixed bug when compiling

Fixed higen example removing obsolete attribute in CIA402 node params file

Fixed controllers for working with latest version of xmlclass

Added bug in support for set flag for optional variables activation in axis public struct

Added support for set flag for optional variables activation in axis public struct

Replaced location for axis ref from %IW(location).0 to %IW(location).402. Added location for network position at %IW(location).

Rewrite CIA402 node specific code generating part

Cherry-pick and re-commit to legitimate ancestor of commit 'Ethercat Management Function Refactoring Source by RTES Lab.' from youcu <youcu1022@gmail.com>

Fixed bug when drag'n dropping variable in master startup command grid

Moved ratio scaling back to MCL. Rewrote CiA402 C part in a way it can be read and understood by some humans.

Added icon for Ethercat extension root and Ethercat master node

Fixed bug with global instance declaration

Fix bug in when moving Ethercat slaves fixed

Added mapping for TouchProbe function in CIA402 slave node

Fixed bug with axes boolean parameters value

Added error message when a device informations can't be found in ESI files

Fixed ToolTip on VariablesGrid columns on Windows

Update higen drive example introducing usage of csv mode

Merged

Fixed bug in support for module extra params in EthercatCFileGenerator

Added support for module extra params in EthercatCFileGenerator

Fixed support for multiple module extra params in ModulesLibrary

Added support for multiple module extra params in ModulesLibrary

Added support for using CIA402 motion mode csp, csv and cst

Fixed order of LocationCFilesAndCFLAGS, master was called after slaves in PLC init and retrieve functions

Fixed bug on Windows with sizers on Windows in Master Editor panel when its size changed

Fixed Save As... function in Beremiz

Added support for stopping SDOThreadProc when stopping PLC

Fixed bug when modifying slave module type

Fixed bug in filters behavior on Windows

Fixed filter behavior in MasterEditor and SlaveEditor

Fixed CIA402 node template for working with MC_Power modification removing implicit set position

Fix scan network functionality, project tree not refreshed after adding slaves

Rollback last commit

Fixed CIA402 node template for working with MC_Power modification removing implicit set position

Fixed bugs in drag'n drop from variables panel in master and slaves editor panel

Added initialization of abort_code and result_size to prevent warning message if not used in following code

Fixed header files inclusion in CIA402 node template file

Fixed (again) some non-clean exit in case of SDO failure at init

Merged

Fixed ethercat test with higen drive project

Fixed bug when using real axis and simulated axis at the same time (got the same index)

Now failure in template based part of C code also release the master for later use...

Better logging, saffer failure at init (allows restart)

Added error logging when SDO FB fails

Added logging based on collecting Kernel logs

Fixed typo

Fixed linking with non RTDM etherlab lib. Now with rtdm...

Split etherlab.py into multiple files

Fixed bug with process variables grid