SVGHMI: Simplification and optimization. func:parselabel becomes a template.

All labels are parsed into a global variable containing resulting widget
description as a node-set. func:widget(id) was added for random access to
widget description.

<?xml version='1.0' encoding='utf-8'?>
<BaseParams xmlns:xsd="http://www.w3.org/2001/XMLSchema" IEC_Channel="0" Name="py_ext_0"/>