Gio::ActionMap: Fix a reference-counting problem.

* gio/src/actionmap.hg: add_action_vfunc(): Add a custom _CONVERSION()
  that takes a reference, just for this vfunc. We would have
  seen the need for this before if it had used a const & instead
  of passing the RefPtr by value, which we probaby cannot change
  now without breaking ABI.