Per-desktop overrides aren't limited to keys in org.gnome.mutter, so
we can use them instead of the alternate-tab extension to default to
the window switcher in the classic session.
https://bugzilla.gnome.org/show_bug.cgi?id=786496
GSettings now recognizes per-desktop overrides that can be used
to change schemas' default values for classic mode, so use that
instead of the separate override schema we currently use with
mutter's custom override mechanism.
https://bugzilla.gnome.org/show_bug.cgi?id=786496