SVGHMI: Extend widget labels parsing to allow expressing widget's accepted types in labels

SVGHMI: Add analyse_widget stylesheet and python code to execute it, in order to obtain widget signature independently of DnD SVG file generation.

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...