For Wayland windows we can have a sequence of window unmapped
(windowHidden signal) followed by a windowClosed way later when the
application quits. This is for example the case with menus.

The result of this was that the fade out animation triggered when the
application quit showing all the already closed menus again.

This change implements a windowShown and windowHidden handler and
triggers the fadeIn/Out animation on it. If the window gets shown
again the existing fadeOut animation gets cancelled, so that it can run
again.

If a window gets closed for which a fade out animation has been run
already, it's not triggered again, thus ensuring that we don't see
zombie windows.

CCBUG: 372622