Use the OSX menubar layout for XFCE, Unity, etc.
Rename menus-osx to menus-traditional so that it will automatically get loaded on platforms that require traditional menubar layouts. Add "Quit", "Preferences" and "About" items in the appropriate places, but mark them as hidden on Mac OS. Simplify configure.ac to let GtkApplication handle things automatically. We now also need to deal with the case where the gear menu will not exist, even on non-OSX systems. Do that by hiding the button. Based on a patch by Ryan Lortie <firstname.lastname@example.org>. https://bugzilla.gnome.org/show_bug.cgi?id=741904
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