Replace custom override schema with per-desktop override

GSettings now recognizes per-desktop overrides that can be used to change schemas' default values for a particular desktop. This is not entirely unlike our existing custom override mechanism in mutter, except that it is not limited to keys in org.gnome.mutter, and it doesn't require a separate schema - the latter means that we (and gnome-teak-tool) no longer have to figure out the correct schema for the current login session and just use the original one.

https://bugzilla.gnome.org/show_bug.cgi?id=786496

Marking as WIP as the required glib API hasn't landed yet.