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).
Put this in a separate file and substantially refactor it.
Move handling of submenu creation into gtkmodelmenuitem where it
belongs.
Improve our handling of when to show separators or not.