Returns an array of nsIEventListenerInfo objects.
If aEventTarget doesn’t have any listeners, this returns null.
Returns an array of event targets.
aEventTarget will be at index 0.
The objects are the ones that would be used as DOMEvent.currentTarget while
dispatching an event to aEventTarget
@note Some events, especially ‘load’, may actually have a shorter
event target chain than what this methods returns.
Returns true if a event target has any listener for the given type.
Add a system-group eventlistener to a event target.
Remove a system-group eventlistener from a event target.