SVGHMI: Added widget_desc to declare widget signature and description

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.

SVGHMI: keep track of order of selection in HMI tree, so that variable can be passed in same order to DnD widget

SVGHMI: fixed typo

SVGHMI: still quite naive path substitution whn prepearing widget for DnD, but now uses label generation.

Multiple widget DnD and mutiple variable still make no sense because all path are replaced with same path, but atleast min/max value are preserved...

SVGHMI: Added text box for tranformation feedback aside widget mignature

Merged default in SVGHMI

SVGHMI: prototype of widget interface description using SVG's desc field, added to SVGHMI's internal svg widget library.

SVGHMI: Naive implementation of widget path substitution in widget DnD: considers widget always have only one variable, and no type checking.

SVGHMI: Widget transform before DnD now have HMI path as a parameter

SVGHMI: Widget transform before DnD now should have HMI path as a parameter, but this path isn't computed for some reason... WIP.

SVGHMI: Widget DnD to Inkscape : Added source SVG widget label parsing and pass selecte HMI subtree to XSLT tranform, so that SVG containing multiple widgets can later be matched against hmi tree fragments, in order to DnD complex groups of widgets.

SVGHMI: Widget Library Picker now transforms SVG widget before allowing DnD. Transform is just identity forn now, but label parsing have already been included. To be continued.

SVGHMI: Ensure that when widgets are nested in HMI:Switch widget, their sub-elements are taken as switch's choice in case their label match the syntax.