If a section or submenu item has a "action-namespace" attribute, the
action names of the created GtkModelMenuItems will be prefixed with that
namespace. Namespaces can be cascaded.
Drop the explicit passing of GActionGroup into the GtkMenu(Bar)
constructors and operate from the action context instead.
With GtkMenuItem implementing GtkActionable, this turns out to be pretty
easy (and most of the code can be removed from GtkModelMenuItem,
including the GActionObserver implementation).
This GtkMenuItem subclass (and GActionObserver implementation) contains
all the knowledge necessary for converting a GMenuModel item description
into a GtkMenuItem.
Remove much of the code that used to do this from
gtkapplicationwindow.c.
