SVGHMI: In order to allow widget signature and description to coexist in same ysl2 file, introduced widget_class, widget_defs to declare widget codegen templates and gen_index_xhtml to mark templates that are only usefull in gen_index_xhtml.xslt.

<?xml version='1.0' encoding='utf-8'?>
<BaseParams xmlns:xsd="http://www.w3.org/2001/XMLSchema" IEC_Channel="2" Name="py_ext_0"/>