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...