SVGHMI: drop useless code from gen_dnd_widget.yslt2, renamed python callback to pass messages

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.