Restore some method .defs.

* clutter/src/clutter_methods.defs: Manually add back some
  clutter_behaviour_*_new() methods that h2defs.py is not
  recognizing now for some reason.
clutter_signals.defs: Manually add a const to fix the build.

The C header now uses a const for the signal's default signal
handler's parameter, but the GType signal cannot represent that.