Add support for Modbus (TCP and RTU) working as master & slave

merge

fixed erro introduced 2 commits ago (-e) : tuples have no reverse() method

Trying to get better logging/display of exceptions at runtime's startup

Changed way to deal with extensions arguments (-e) so that path and filename are stored independently and accessible for extensions themselves.

Textviewer : moved call to Colourise in a better place, so that ST highlighting gets also updated when typing in some case, not only IL

Main runtime template C code : Added HAVE_RETAIN preprocessor definition for customized build to signal it provides IEC-61131 Retain memory handling function. Removed targets/Xenomai/plc_Xenomai_noretain.c, now useless.

Clarify licensing, and packaging of runtime only files :
- moved and split targets/typemapping.py into runtime/typemapping.py and runtime/loglevels.py
- added runtime_files.list pointing to files necessary for runtime, and referring to runtime license

Enforced check_source.sh pep8 and pylint rules.

runtime/WAMP: Fixed secret loading into configuration. Fixed secret commandline switch handling.

runtime/WAMP : was 'Added time print on session left.' from dporopat. Purpose is to log (dis/re)connection, to help debugging connection problem

runtime/Wamp : adapt to unicode being enforced in latest crossbar API. Updated some documentation, to explain why there is unused definitions like SubscribedEvents or DoOnJoin.

Extend python<->PLC global variable description "GlobalDesc" to support Initial value setting.

Rebased from dporopat change "add initial value to runtime.py file", which means :
- GlobalDesc is used by some customized beremiz python runtime
- Those runtimes need to know what is initial value of these special variables

update tests/wamp README file, based on changes from dporopat.