xcb: Only create listener window when needed

The xcb QPA plugin uses a separate thread for getting events from the
X11 server in a race-free manner. This thread has to be stopped when
shutting down.  This is done by causing the X11 server to send a special
event that causes the thread to exit when it sees it. (Also, the thread
exits if the xcb connection goes into an error state, but that does not
matter here)

So this event listener window is only needed when Qt is sending events
to itself. This means that it does not have to be kept around and
instead can be created only when needed.

Since this window does not stay around for long, it does not need a
name. However, it does need the list of screens to find the correct root
window, so destroy screens later.

Change-Id: Ib4104cbc3f372fa5d1391bcf5a633a158409862f
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>