extIEvents Interface

 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.

 @param   aEvent
          The name of an event
 @param   aListener
          The reference to a listener

void addListener(in AString aEvent, in extIEventListener aListener)

 Removes an event listener from the list. Calling remove
 with arguments which do not identify any currently registered
 event listener has no effect.
 @param   aEvent
          The name of an event
 @param   aListener
          The reference to a listener

void removeListener(in AString aEvent, in extIEventListener aListener)