rename text field in SVG graphic to match IEC code in svgui example

This caused svgui example to crash.

disallow setting IEC location addresses for TIME, DATE, DT and TOD variables

Size of these types is not defined and depends on the implementation
(IEC 61131-3 2.3.1).

if there is no users matiec installation, then use system path to run iec2c

search for matiec libraries in system path as well

Closes #29

remove child node from add child node menu, if max count of the same type nodes reached

fix reducing groups with ordered elements

fix extra brace

add getCompiler and getLinker methods, to abstract from XSD.
add calc_source_md5 as it's more convenient especially for embedded
targets to add -DPLC_MD5="something" option at compile time than
to mess with editing binary.
add some more helper methods

make run Beremiz and PLCOpen Editor, if full path contain non-lating
characters

There was a problem in About dialog and in project properites tab and
probably in my other places, when str is automatically converted to
unicode string using ascii.
This patch converts str to unicode using proper encoding at the very beggining.

README.md edited online with Bitbucket

fix clearing project name on opening project properties page.

The problem exists only on Windows with wxPython-3.0.

This happens because callback for EVT_KILL_FOCUS for a first text
control (project name) is called just after creating page with project
properties. It happens before initializing text controls from PLCController.

fix problem with debugging external string variables

Debug update stopped if external string variable added to debug
variable panel.

fix problem with clearing non-empty location field after changing type of config variable

introduced by cd9db17 ("clear variable location cell in variable
panel, if variable class or type changed").

Closes #26

fix backtrace on opening program/resource on small screens

Fix regression introduced by f4696d8 ("add stretching to columns Name,
Type, Doc in variable panel").
This problem happens only on wxPython-3.0.

show whether instance of SFC action is currently active or not in its
debug instance view