Show unmanaged windows in composite. *Stacking will be broken though still!!*

Show unmanaged windows in composite. *Stacking will be broken though still!!*

adds window_cleanup() that subclasses of ObWindow call to tell the parent they are going away, allowing it to clean up using the abstract pointers.

unmanaged windows get OB_STACKING_LAYER_ALL, but the stacking code is not made to handle this value yet.  unexpected behaviour may result !