workspace: Remove tweens when hiding overlays

workspace: Remove tweens when hiding overlays

The window overlays may be shown erroneously if hideOverlays is
called while the corresponding clone has an uncompleted tween which
calls showOverlays in its onComplete handler, for instance when
quickly leaving the overview before the initial overview animation
has finished. To fix, remove all existing tweens when hiding the
overlays.

https://bugzilla.gnome.org/show_bug.cgi?id=666020