SVGHMI: changes that should fix some corner cases about unlinking and discardable element that still didn't show until now.
- list that would have been declared in some groups could have been discarded.
- elements to be unlinked were comuted based on filtering $hmi_elements, whereas $hmi_widget already filtered list

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<?xml version="1.0" encoding="UTF-8"?>

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<BeremizRoot URI_location="LOCAL://">

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  <TargetType/>

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</BeremizRoot>