Replace override schema with desktop overrides
See mutter!159 (merged) and gnome-shell!46 (merged) for the related mutter and gnome-shell changes.
This removes the alternate-tab extension, so to avoid a flamefest, we need to communicate that there's no loss of functionality before hitting that green button ...
Edit: I dropped the alternate-tab removal from the patch set, so this should now be uncontroversial.
Edited by Florian Müllner