full ICCCM 2.0 WM_S%d selection compliance

 - ICCCM 2.0 requires window managers to take ownership of the
   WM_S%d MANAGER selection where %d is the screen number, waiting
   for any previous owner to exit and then sending a MANAGER client
   message to the root window identifying the selection

 - Also, when another window manager take the selected (a Selection
   Clear event is received for WM_S%d) the window manager cleared
   must exit.

 - This commit provides JWM compliance to ICCCM 2.0 in this respect.

 - Tested.
Use only a single window as indication that JWM is running.