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

<?xml version="1.0" encoding="UTF-8"?>
<BaseParams Name="wxglade_hmi" IEC_Channel="0"/>