QWinTaskbarButton::progressBar() -> progress()

The class is called QWinTaskbarProgress -> use the same name for
the accessor. That name was also used for the QML grouped property
name since it looks really nice. :)

Change-Id: I6c46ea4336dd3f420b16a305de47b490f7629adb
Reviewed-by: Ivan Vizir <define-true-false@yandex.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>