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
This simplifies the code required to build remote menus and
put all the items in the right place, and makes us share our
implementation with GTK+.
https://bugzilla.gnome.org/show_bug.cgi?id=698427
