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.

{
    "ID": "wamptest", 
    "active": true, 
    "protocolOptions": {
        "autoPingInterval": 60, 
        "autoPingTimeout": 20
    }, 
    "realm": "Automation", 
    "url": "ws://127.0.0.1:8888"
}