SVGHMI : HMI_STRING now also supported from HMI to PLC

SVGHMI : cosmetic

SVGHMI non significant fixes

SVGHMI : support for HMI_STRING and HMI_BOOL

plc_debug.c/var_acces.c : whitespace cleanup and other cosmetic changes

SVGHMI : Added python fomating {port} and {name} to commands so that command can build target URL

SVGHMI : add on Start, Stop and Watchdog command fields to configuration

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.