SVGHMI: split page switch into switching subscription and switching elements in the DOM, to ensure that subscriptions have been send before changing DOM, and avoid some flicker.

<?xml version='1.0' encoding='utf-8'?>
<ModbusServerNode xmlns:xsd="http://www.w3.org/2001/XMLSchema" Local_Port_Number="1502" Local_IP_Address="127.0.0.1"/>