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