scene: Don't hide closed windows

If a closed window is kept alive, it means that somebody needs to
animate it and therefore it should be visible. Otherwise the window
would be destroyed and its item would be removed from the scene.

This change makes the WindowItem not change its visibility if the
associated window is closed. If the window had been invisible before
it was closed, the item would be invisible; if the window had been
visible before it was closed, the item would be visible.