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.

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.

SVGHMI: Code cleanup for ToggleButton.

SVGHMI: Fixed last i18n changes meant to allow checking changes in PO files. Was making build exception when no PO files available.

IDE: Add a shortcut to project "Clean" action : F9

merge

SVGHMI: fixed embedded fonts family name not matching when using windows version of inkscape

SVGHMI: force svghmi build if any change in fonts or translation (.PO) files

SVGHMI: Add file type drag'n'drop to widget picker, for now drops widget's original SVG.

SVGHMI: silence debug output in generated xhtml.

SVGHMI: Implemented "Add Font" and "Remove Font", add font embedding in CSS at build time, tested ok with some OTF for now.

SVGHMI: Add font management buttons and icons, doing nothing for now

Fixed misuse of GridCellEditor in DurationCellEditor, potentially leading to crash on windows.

SVGHMI: Fix update of HMI tree in UI when loading from XML at start. Removed some dead code left after split of svghmi.py into svghmi.py+ui.py.

merge

SVGHMI: added "page_node" variable that reflects the HMI tree path of current relative page

SVGHMI: Add list of HMI tree path in a JS array, so that we can use it for multiple purpose :
- display name of edited variable in keypads (currently wrong in relative pages)
- have some variable that hold basename of current relative page HMI_NODE's path
- propose some list of names in DropDown when selecting a relative page to jump to, without having to instancite many jump widgets.