* An Event whose processing may be interrupted when the event has been handled.
* A Dispatcher implementation MUST check to determine if an Event
* is marked as stopped after each listener is called. If it is then it should
* return immediately without calling any further Listeners.
* Is propagation stopped?
* This will typically only be used by the Dispatcher to determine if the
* previous listener halted propagation.
* True if the Event is complete and no further listeners should be called.
* False to continue calling listeners.
public function isPropagationStopped() : bool;