Stage: Deprecate get_is_default() and avoid deprecation warnings.
clutter_signals.defs: Add and use a patch.

* clutter/src/clutter_signals.defs.patch: Add this patch that
  makes the necessary manual changes.
* codegen/gen_scripts/clutter_generate_extra_defs.sh: Use
  the patch after generating clutter_signals.defs.
  gtkmm does this too.