Add GSettingsBackend API to allow any backend to have a 'has-unapplied'
property. Remove the 'owner' hackery that was used for notifying of
changes to 'has-unapplied' and just use normal GObject notify (and wire
into that from the frontend).
Drop the 'vtable' business and switch to a single callback that takes a
new GSettingsEvent structure.
This patch temporarily regresses delayed settings: if a key set within a
delayed settings backend becomes non-writable, it is no longer removed
from the changeset.
Since #include <gsettingsbackend.h> is a perfectly valid thing for
applications to do, and since we want to include gio headers from
gsettingsbackend.h, we need to effectively disable the #error we would
get from those headers (because we're not coming via gio.h).
We don't want to #include <gio/gio.h> here because this would cause
needless rebuilding of GSettingsBackend, GSettings,
GDelayedSettingsBackend, etc... every time someone changed anything in
any public header.
