Interface for classes capable of having callbacks to handle events.
| HandleEvent | Handle an event, most often by executing the callbacks associated with the event. |
| HasBubbleHandlers | Return true if event handlers for the event propagation bubble up phase have been attached on this object. |
| HasCaptureHandlers | Return true if event handlers for the event propagation capture phase have been attached on this object. |
| OnLostCapture | Called when the element loses the capture. Will be removed in a future version. |
| RegisterCallback | Add an event handler on the instance. If the handler has already been registered on the same phase (capture or bubbling), this will have no effect. |
| UnregisterCallback | Remove callback from the instance. |