SVGHMI: Add arbitrary variable assignment when entering Pages

Usage similar to Assign widget.
Required instanciating PageWidgets for each page, so that assigned variables can be subsribed.

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<?xml version='1.0' encoding='utf-8'?>

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<BeremizRoot xmlns:xsd="http://www.w3.org/2001/XMLSchema" URI_location="LOCAL://">

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  <TargetType>

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    <Linux/>

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  </TargetType>

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</BeremizRoot>