SVGHMI: ensures that PLC sends only fresh data to HMI, even right after variable being subscribed.

OPC-UA: add logging to help user to understang what went wrong when connecting.

OPC-UA: fix LDFLAG, missing OpenSSL's -lcrypto

open62541 is linked staticaly, therefore shared object dependencies have to be given explicitely.

tests: missing SSL support in open62541 build parameters

Merge changes from default branch

merge

Merge pull request #5 from vitw/wxPython4

update installation instructions

docs: add additional dependencies to installation instructions

update installation instructions

SVGHMI: fix svghmi_jumps example's SVG file (viewport was misplaced)

SVGHMI: rework the way cached values are applied to widgets when fresh data is missing.

SVGHMI: re-arrange Animate and data Dispath code paths.

SVGHMI: add an exemple that demonstrate use of Jump widget in different use cases.

SVGHMI: refactor an factorize widget dispatch() method calling

SVGHMI: fix ToggleButton not always reflecting current state

IDE: fix exception transfering or reconnecting after tracing variables

SVGHMI: Add predefined assignments to jump widget to notify PLC or other widgets about jump.

SVGHMI: continue rework of Jump widget to cope with Enable Expressions.

Questioning widget's activity and disability all together.
Now active/inactive/disable masquarade is all managed in widgets_common.
Propagated changes to widget already using activity (buttons)

SVGHMI: rework Jump widget to cope with Enable Expressions.

Relative jumps are not disabled implicitely anymore when pointed HMI_NODE is 0.
Also, "activable()" implementation was enhanced to make it optional.

SVGHMI: add support for "enable expressions" with arbitrary variable name assignment.

HMI tree paths can be prefixed with a variable name "@name=/MY/HMI/VAR"
Widget declarations can end with a "#" followed by a JS expression that refers to name given to variables.
Widget is disabled if expression's result is false.

Commit includes some more-or-less related generated code refactoring, that should simplify extending widget's variables attributes.

SVGHMI: preliminary implementation of a general enable bit for all widgets, using "#" at the end of widget declaration, and a "disabled" element that becomes visible instead of widget when disabled.

SVGHMI: rename veriables in parse_labels.ysl2 for better readability

OPC-US client: add support for x509 and user+password authentication.

Commit for backup not including tests since encrypted OPC-UA test are still not valid.

IDE: fix wxPython 3->4 API change leading to exception in type selection menu in CodeFileEditor

OPC-UA, IDE: add CTN parameters to support OPC-UA encryption and authentication.

xmlclass had to be enhanced to support elements with no content in xsd:choice

IDE: Fix tooltip not being shown anymore on any toolbars since switch to wxPython4.

Cleanup and update version.py

IDE: Fixed About dialog exception on Windows, and typo in about dialog's content.

runtime: fix bugs introduced in previous commit, and refactor some code

Runtime: wx.app.mainLoop and twisted reactor now share main thread with runtime.Worker.

This fixes exception when invoking python interactive shell from Runtime's tray
icon right-click menu.

Probably a consequence of wxPython upgrade, together with initial bad idea to
move wx.app.mainloop in non-main thread.

Remove useless assignment

SVGHMI: update generated XSLT