Moving a mention from one cluster to another cluster is not supported yet (but it seems @dan-zeman would like to use it). We should:

• remove the NotImplementedError in the setter
• add self._cluster._mentions.remove(self)
• decide if cluster._mentions should be always sorted (and if yes, add a sort())
• if self._bridging and new_cluster in self._bridging.targets: raise ValueError("Bridge cannot self-reference the same cluster")
• make sure there are no other hidden problems