Notifies all listeners that have registered interest for
notification on ZGroupEvents that a child has been added to the
group, then percolates the event up scenegraph, notifying any
listeners on any higher ZGroup nodes.
Notifies all listeners that have registered interest for
notification on ZGroupEvents that a child has been removed from the
group, then percolates the event up scenegraph, notifying any
listeners on any higher ZGroup nodes.
Compute the bounds of this handle using the handles locator to determine
the center point of the bounds and the DEFAULT_HANDLE_SIZE to determine the extent.