Emit QWinTaskbarProgress::minimum/maximumChanged()

Ensures that the progress indicator turns correctly to an indeterminate
indicator when minimum and maximum are both changed to 0.

Change-Id: Ibe3ba98f92f80a462983e6894b60029c8a4200c6
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Add tst_QWinTaskbarProgress

Change-Id: I4ba748561aef6597762f52d458e951112255f0de
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>