SVGHMI: changes that should fix some corner cases about unlinking and discardable element that still didn't show until now.
- list that would have been declared in some groups could have been discarded.
- elements to be unlinked were comuted based on filtering $hmi_elements, whereas $hmi_widget already filtered list

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<?xml version='1.0' encoding='utf-8'?>

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<SVGHMI xmlns:xsd="http://www.w3.org/2001/XMLSchema" OnWatchdog="echo Watchdog for {name} !" OnStart="chromium http://127.0.0.1:{port}/{name}" OnStop="echo Closing {name}" WatchdogInitial="10" WatchdogInterval="5"/>