Transform a pending repaint into a workspace repaint before destroying Deleted

The sliding popups effect schedules a repaint and then unreferences the
deleted window. The problem with doing so is that the scheduled repaint
will be effectively discarded because the Deleted will be destroyed once
we are back in the event loop.

This issue is most noticeable on Wayland. Not sure why. If you close
Kickoff, you may see its flickering ghost in background.

If it happens that a Deleted has a pending repaint, transform it into a
workspace repaint to avoid discarding any scheduled repaints.