Add extra .defs for documentation generation.

* clutter/src/clutter_extra_objects.defs:
* clutter/src/filelist.am: Add this extra .defs file just to tell
  the documentation generator about interfaces that otherwise
  would not show up in the .defs files.
* clutter/src/clutter.defs: Include the new .defs file.
* codegen/extradefs/generate_extra_defs_clutter.cc: Add a type
  here too, also for the documentation generator.
* clutter/src/clutter_signals.defs: Regenerate, keeping the
  hand-coded changes.