GSettingsBackend: move 'has-unapplied' logic here

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).

gio/gsettingsbackendinternal.h

@@ -71,6 +71,8 @@ G_GNUC_INTERNAL
 GPermission *           g_settings_backend_get_permission               (GSettingsBackend     *backend,
                                                                          const gchar          *path);
 G_GNUC_INTERNAL
+gboolean                g_settings_backend_get_has_unapplied            (GSettingsBackend     *backend);
+G_GNUC_INTERNAL
 void                    g_settings_backend_sync_default                 (void);
 
 G_GNUC_INTERNAL