gck: Enumerator can now retrieve object attributes

gck: Enumerator can now retrieve object attributes

 * New interface called GckObjectAttributes. Callers derive from
   GckObject, implement GckObjectAttributes and set fields in the
   iface to denote which attributes are interested in.
 * Caller passes type of derived object to gck_enumerator_set_object_type()
   and then enumerator will retrieve attributes set in iface, and return
   objects of that type
 * GckObjectAttributes has attributes property.