SVGHMI: add a way for widgets and other yslt2 files to add css definitions to resultng page.
<?xml version='1.0' encoding='utf-8'?>
<BeremizRoot xmlns:xsd="http://www.w3.org/2001/XMLSchema" URI_location="LOCAL://">
<TargetType/>
<Libraries Enable_Python_Library="false"/>
</BeremizRoot>