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

<?xml version='1.0' encoding='utf-8'?>
<MemoryArea xmlns:xsd="http://www.w3.org/2001/XMLSchema" MemoryAreaType="03 - Holding Registers" Nr_of_Channels="1"/>