XSLTransform.py: added get_error_log()

Attempt to workaround problem reported about empty plc.xml after unrelated crash. Now, if etree model is corrupted in a way tostring() would generate an empty string, exception prevents saving.

docsvg.py : Better error message when inkscape is not installed.

Fix unmatched "Courier" font for monospace fonts on latest ubuntu 20.04. Selected "FreeMono" instead, present since at least 18.04

Python Runtime: order of execution of extension's init() and cleanup() now reflects order of appearance of extensions in configuration tree.

Runtime: change PLCObject 'BlobAsFile' method to allow customization by overriding

WebInterface: better wording for wiping PLC content: "Repair"

runtime: added restart/reset button in web settings. Accidentaly fixed web settings logging feature.

Add a little test to see OnChange's .count .first .last in action

Fixed "Onchange" object added in previous commit

Python Safe Globals now have more reliable triggering of OnChange call. Added "Onchange" object to accessible runtime variables that let user python code see count of changes and first and last values.

Project tree: avoid flickering and glitches in notebook when selecting some still not opened ConfigTreeNode

Project tree: Clicking on ConfigTreeNode should have same behaviour as for POUS and other IEC types : simple click to select and double click to open.

py_ext: better handling of coma separated OnChange attribute for python safe global variables.

Allow customization of generated py_ext OnChange calls, by adding a static method to PythonFileCTNMixin that can the be later hot patched.

PY_EXT: OnChange is now a coma separated list of callables. Updated tests/python accordingly.

Modbus: fixed typo in ctype typing.

MODBUS: Fix saving of websettings.

Runtime/PLCObject : move call to PreStartPLC right after call to the "start" method of all python extensions, and then just before the start of the python loop executing python_* blocks.

Modbus plugin: reformat code, and add comment.

Modbus plugin: fix runtime periodic timer (don't use POSIX timer)

Modbus and Bacnet websettings : Rename variables and functions to avoid name collisions.

Websettings for modbus and bacnet are now passed to runtime as python files loaded (execfile) at init of PLCObject with the same globals.
Because if this, same names used in previously different modules now colide.

Modbus: fix strange side effect of re-use of condition variables or mutex with Xenomai : without bzero of the associated types, mutex keeps locked.

Modbus: fix client timer invalid arguments.

Modbus add a timer thread, and switch timer to SIGEV_THREAD_ID to follow xenomai posix skin restrictions.

Runtime: Fix typo in PLCObject leading to exception when reporting failure.

Py_ext: use non-derived type when generating C code for handling Python PLC globals.

tests/python : add some STRING PLC Python globals with OnChange.

Runtime: STRING type for Python PLC Globals was making problem with strings, since generated code wasn't doing comparison properly.

Merge