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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<BaseParams xmlns:xsd="http://www.w3.org/2001/XMLSchema" IEC_Channel="0" Name="svghmi_0"/>