Scans through list of keyframes in the channel and removes those
        which are not in self.key_frame_list.

def del_unused_keyframes(self):
        """Scans through list of keyframes in the channel and removes those
        which are not in self.key_frame_list."""
        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]