Enqueue updates that are triggered while an update is applied.

When an update is applied, additional updates may be triggered during
listener notification. To ensure that all subscribed listeners have seen
model changes caused by the current update before the next update will be
applied, updates triggered by listeners will be deferred until all
subscribed listeners have been notified.
Copy the listeners before notifying them.

Added or removed listeners are to be ignored during notification.
Changed misleading function name.