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.