* clutter/clutter-script-private.h: Add a flag for the
default stage.
* clutter/clutter-script.c:
(json_object_end): If the "type" member is "ClutterStage"
and we have a "is-default" member set to true then this
is the default stage.
(clutter_script_construct_object): Special case the default
stage instead of each ClutterStage.
(object_info_free): Ditto as above.
* tests/test-script.json: Test the creation of a non-default
stage and the ::destroy handler to quit.