- GEnum type for G_PRIORITY_VALUES?
- can't implement g_idle_remove_by_data because ... well, how would you
  search for the data value?
- proper cleanup for bad parameter types in Glib::Object::new?
- GValue.xs may require the ability to register handlers for new fundamental
  types, since gperl_sv_from_value and gperl_value_from_sv croak on unknown
  or unimplemented data types.
  - gperl_value_from_sv should change to void return since it croaks on error.
- perl methods to list possible GEnum and GFlags methods
