Remove a registered observer (listener)
void Unregister( CXlEventObserver<T>* observer );
A pointer to an observer of events of the appropriate type, which was previously registered using CXlEvent::Register(). The observer will no longer receive updates when CXlEvent::NotifyObservers() is called.
Note that the lifetime of the observer is entirely under the observer's control; the event does not "own" the observer, and it does not destroy the observer when the observer is unregistered. It is the repsonsibility of the developer to ensure that the lifetime of the observer is greater than its period of registration.