effects/slide: Ensure that there's only one visibility ref per window

If a window is added and then the current virtual desktop changes, we
can encounter the following situation:

 * desktopChanged signal is emitted, and the slide effect starts
   animation. SlideEffect::prepareSwitching() will setup windows; note
   that the new window may be present in effects->stackingOrder()
 * windowAdded signal is emitted, and slide effect tries to ref the
   window again

In order to ensure that there's only one reference, maintain visibility
refs in a hashtable.

BUG: 455237


(cherry picked from commit 1725e224)