Interface for supporting custom events.
Adds an event listener to the list. If multiple identical event listeners
are registered on the same event target with the same parameters the
duplicate instances are discarded. They do not cause the EventListener
to be called twice and since they are discarded they do not need to be
removed with the removeListener method.
| aEvent | The name of an event |
| aListener | The reference to a listener |
Removes an event listener from the list. Calling remove
with arguments which do not identify any currently registered
event listener has no effect.
| aEvent | The name of an event |
| aListener | The reference to a listener |