WIP: Add skeleton for MQTT extension based on part of OPC-UA extension.
For now generated code makes no sense but persistently-configurable-C-generating-extension infrastructure is there.
<?xml version='1.0' encoding='utf-8'?>
<SVGHMI xmlns:xsd="http://www.w3.org/2001/XMLSchema" OnWatchdog="echo Watchdog for {name} !" WatchdogInitial="10" WatchdogInterval="5" EnableWatchdog="true" Path="{name}"/>