[AD] Removing the default close button support

[Eric Botcazou <ebotcazou@xxxxxxxxxx>]

As already explained many times, I think we should get rid of the default 
behaviour of the close button, i.e shutting down the program without the 
express consent of the primary thread.

This would imply that the close button is disabled until the user registers a 
close button callback function, so we don't need two functions anymore in 
order to handle it.

Therefore I propose to get rid of both set_window_close_button() (useless now) 
and set_window_close_hook() (nasty name: Allegro uses callbacks, not hooks) 
and add set_close_button_callback(). For backward compatibility, 
set_window_close_button() would be a dummy function and 
set_window_close_hook() redirected to set_close_button_callback().

-- 
Eric Botcazou