meldwindow: Manually handle app menu for non-Gnome shells (bgo#725430)

Since we don't have a global application menu bar (due to limitations
of GMenuModel, and having to totally redesign all of our menus and
rewrite associated code because of GtkUIManager deprecation), this
commit adds a fallback that manually recreates the application menu
in a UIManager-compatible form, and inserts it into the appropriate
(new) placeholder.