To avoid recreating the app menu unnecessarily, the panel checks
whether the menu's current actionGroup already matches the target
one. However as the menu's actionGroup property is currently private,
the test always fails, whoops.
https://bugzilla.gnome.org/show_bug.cgi?id=781471
Wrap new GtkMenuTracker API that adds an easy way to bind to
tracker items, and use it to add back support for submenus.
This also adds support for a submenu feature that we didn't
have support for before, action namespaces.
https://bugzilla.gnome.org/show_bug.cgi?id=700257
This pulls in new upstream API that Ryan will maintain, removing
code on our side.
Currently, our implementation of submenus will be gone, but this
will be fixed in a few commits.
https://bugzilla.gnome.org/show_bug.cgi?id=700257