Finished fixing Wx IDs abuse. There was still some wasted IDs because of wx.NewId calls in many places where it wasn't needed, and those IDs were not re-used. As a consequence Beremiz was making exception crashing after a few hours of intensive use.

SVGHMI: DnD UI : changed layout again, moved description in a vertical splitter, added scrollbar for preview and signature, fixed flickering description

SVGHMI: UI: Swap widget selection tree and preview panel

SVGHMI: WIP: Widget Library UI: Reworking widget selection and binding.

Merged SVGHMI branches

SVGHMI: WIP on Widget DnD UI : Added documentation to widgets, that is injected in widget parse tree during widget analysis

SVGHMI: Fix behaviour of Jump widget, that wasn't displaying "disabled" state in some cases.

merged default in svghmi

SVGHMI: updating generated XSLT

SVGHMI: Intermediate state while updating UI, preparing for displaying widget description and multiple variables selections

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.