SVGHMI: add CURRENT_PAGE_{location} global variable to reflect currently visible page. If PLC wites some valid page reference in that variable, it triggers page switch. Additionally, fixed /HEARTBEAT being subscribed systematically by JS code even when wtchdog is not enabled.

<?xml version='1.0' encoding='utf-8'?>
<SVGHMI xmlns:xsd="http://www.w3.org/2001/XMLSchema" OnWatchdog="echo Watchdog for {name} !" OnStart="xdg-open http://127.0.0.1:{port}/{name}" OnStop="echo Closing {name}" WatchdogInitial="10" WatchdogInterval="5"/>