QWinTaskbarButton: allow clearing the overlay icon

Change-Id: I3a317404349a5f40263cf9e40b263c701a88f8d3
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Manual test for WinTaskbarButton/Progress

Change-Id: I1d4ca780b10b0cccc01c023b0ee3b033b06a6641
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>