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"?>
<BeremizRoot URI_location="LOCAL://">
  <TargetType/>
</BeremizRoot>