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.

SVGHMI: Fixed wierd CDATA mess in output xhtml, because of backticks copy pasted in some yml ||text block||

SVGHMI: Added fonts.py with functions to convert ttf, otf and woff fonts into data_uri based CSS font-face

merge

SVGHMI: split svghmi.py into svghmi.py (Config Tree Node + code gen) and ui.py (UI for HMI tree and Widget picking)

SVGHMI: stop serving HMI open wide on all interfaces. That was a security hole. It is user/developer responsibity to forward connection with for exemple stunnel and make it securely accessible from LAN.