SVGHMI: add support for "enable expressions" with arbitrary variable name assignment.

HMI tree paths can be prefixed with a variable name "@name=/MY/HMI/VAR"
Widget declarations can end with a "#" followed by a JS expression that refers to name given to variables.
Widget is disabled if expression's result is false.

Commit includes some more-or-less related generated code refactoring, that should simplify extending widget's variables attributes.

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{

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    "ID": "wamptest", 

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    "active": true, 

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    "protocolOptions": {

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        "autoPingInterval": 60, 

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        "autoPingTimeout": 20

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    }, 

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    "realm": "Automation", 

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    "url": "ws://127.0.0.1:8888"

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}