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

          <xsd:attribute name="Compiler" type="xsd:string" use="optional" default="gcc"/>
          <xsd:attribute name="CFLAGS" type="xsd:string" use="optional" default=""/>
          <xsd:attribute name="Linker" type="xsd:string" use="optional" default="gcc"/>
          <xsd:attribute name="LDFLAGS" type="xsd:string" use="optional" default=""/>