Stop using GTK_STOCK_* (deprecated since GTK+ 3.10)
Removed all trace of GtkAction and GtkActionGroup

Also, put some TODO placeholders that we need to implement
to bring the contextual menu and the toolbar back.
Added a toolbar of dummy items
Added XML description for the contextual menu
Make the toolbar items work (associate them to actions)
Bring the contextual menu back, using GMenuModel
Reorganized visibility options (public/private in the same line)