Win32 build: Use Gtk3
Fix incomplete interface localization in Windows build
Missing Translation

N_() seems to be not expended inside ACTION_LAYERSxxxx definition
Brush tool : Autoresizable Brush chooser as Gtk::ToolPalette

* Remplace Gtk::Box by Gtk::Grid as main container (Gtk::Grid is a new widget that'll replace Gtk::Box)
* Remplace Gtk::Grid by Gtk::ToolPalette for brushes container
* Remplace Gtk::RadioButton by Gtk::RadioToolButton
ToolBox : Propagate Scroll Event from Palette

Make the Tool Palette scrollable from inside the palette not just from the scrollbar
Brush tool : Replace RadioToolButton by ToggleToolButton

Active brush was no-efficient + constitency ui with ToolBox

* Convert all RadioToolButton ref to ToggleToolButton
* Remove unuseful Gtk::RadioToolButton::Group references
* Use of StateBrush_Context::selected_brush_button to track previous selected brush
Brush Tool - Fix Automatic created image can't be embed (related to #722)

* Check for filename param
* created #images directory
* save surface
* set layer's filename param and description