code: def del_unused_keyframes(self):
          
          skl = self.key_frame_list.sorted_key_list()
          unused_keys = [k for k in self.dct['keys']
                         if k not in skl]
          for k in unused_keys:
              del self.dct['keys'][k]
docs: Scans through list of keyframes in the channel and removes those
          which are not in self.key_frame_list.