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

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<ModbusServerNode xmlns:xsd="http://www.w3.org/2001/XMLSchema" Local_Port_Number="1502" Local_IP_Address="127.0.0.1"/>