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