GSettings: store (default, options) in gvdb

gvdb just dropped the ability to have a separate "options" field.  We
now store the options into a GVariant along with the default value.

For now, we use a small shim in GSettingsSchema in order not to touch
too much code.  A more complete rewrite will follow.

This represents a change to the schema file format with another likely
to follow.  glib-compile-schemas needs to be re-run after installing
this change.

gio/gsettingsschema.h

@@ -70,6 +70,9 @@ gboolean                g_settings_schema_has_key                       (GSettin
 G_GNUC_INTERNAL
 const GQuark *          g_settings_schema_list                          (GSettingsSchema  *schema,
                                                                          gint             *n_items);
+G_GNUC_INTERNAL
+const gchar *           g_settings_schema_get_string                    (GSettingsSchema  *schema,
+                                                                         const gchar      *key);
 
 G_END_DECLS