/// <summary>
 /// Removes a listener from the event caller
 /// </summary>
 /// <param name="listener">
 /// The listener to remove
 /// </param>
 public void RemoveListener(GuiEventListener listener)
 {
     listeners.Remove(listener);
 }
 /// <summary>
 /// Adds a listener to the event caller
 /// </summary>
 /// <param name="listener">
 /// The listener to register
 /// </param>
 public void AddListener(GuiEventListener listener)
 {
     listeners.Add(listener);
 }