SVGHMI: added a watchdog. To ensure that the whole chain is checked, watchdog use a periodic echo of a hearteat variable. JS client code systematically register /HEARTBEAT at 1s update freq, and reacts on updates of /HEARTBEAT by systematically incrementing it. C code catch /HEARTBEAT update and feeds python-implemented watchdog. For now, watchdog does nothing when tiggered

Add --on-plc-start --on-plc-stop and --status-change to runtime command line, calling given command respectively on start stop or any event. Command line string is python formated (to eventually include status with {}) before being split (supports quoted strings) and passed to Popen for non-blocking execution.

SVGHMI : had to move the problem of wkaing up python thread from plc thread to platform specific code.
Since Xenomai's cobalt thread are definitely incompatible with normal posix python interpreter binary's thread, we must synchronize them with arcane rt_pipes (the only ones that really work cross domain) as already done in debug and python async eval blocks.

SVGHMI: change collect/send thread looping condition to fix infinite loop in some cases

SVGHMI: re-implemented tree view with classic wxTreeCtl

Non significant changes, whitespaces, etc.

SVGHMI: Added simple HMI Tree View.

SVGHMI: add a class attribute to HMI Tree nodes, set when using HMI_NODE

SVGHMI: HMI_LABEL and HMI_CLASS become HMI_NODE.
- Name of parent POU becomes HMI tree node name,
- Name of HMI_NODE variable becomes class of the node.

Add FatalError() method to ConfigTreeNode.
Extensions can report error addressed to user, without a traceback.

SVGHMI: various fixes to make SVGHMI behave on more versions of twisted and GCC.

SVGHMI - prepare page with cached data when switching. This prevents values that do not change and that was already subscribed in previous page from keeping undefined.

SVGHMI: various insignificant code moves, commenting and typos fixes.

SVGHMI : fancier second page in tests/svghmi