SVGHMI: better handling of page switch fade-out.
- In cases or relative jump to current page, page was keeping black after switch.
- Now keep track of undergoing page switch without comparing subscribed page to visible page.
<?xml version='1.0' encoding='utf-8'?>
<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"/>