Add an API to GtkAccelLabel for hardcoding the accel key to be displayed
(ie: allowing us to bypass the GtkAccelGroup lookup).
Use that from the GMenuModel-based GtkMenu construction code instead of
passing around the accel group.
This makes accel labels work in bloatpad again.
This patch effectively removes any hope of automatic runtime accel
changes in GMenuModel-based menus without additional application
support but it leaves the door open for this to be supported again in
the future (if we decide that it's important).
https://bugzilla.gnome.org/show_bug.cgi?id=683738
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.
