Register |listener| to receive |messageName|. All listener
callbacks for a particular message are invoked when that message
is received.
| The message manager holds a strong ref to | listener | . |
If the same listener registers twice for the same message, the
second registration is ignored.
| Undo an | addMessageListener | call – that is, calling this causes us to no | ||
| longer invoke | listener | when | messageName | is received. |
removeMessageListener does not remove a message listener added via
addWeakMessageListener; use removeWeakMessageListener for that.
This is just like addMessageListener, except the message manager holds a
weak ref to |listener|.
If you have two weak message listeners for the same message, they may be
called in any order.
| This undoes an | addWeakMessageListener | call. |