Added add/remove signals to scenes

  Scenes will now signal via their source when an item has been added
or removed from them.

  "add" - Item added to the scene.
  Parameters:  "scene": Scene that the item was added to.
               "item":  Item that was added.

  "remove" - Item removed from the scene.
  Parameters:  "scene": Scene that the item was removed from.
               "item":  Item that was removed.