Created by: jamesobutler
Fixes https://issues.slicer.org/view.php?id=4536
Slicer.util displays (infoDisplay, warningDisplay, errorDisplay, etc) that use messageBox were still visible when hovering over Slicer's Windows 10 taskbar preview even though they were closed. This was causing confusion about whether the displays had actually been closed.
References are now scheduled for deletion when the display is closed.