Grouped WinTaskbarButton.progress property

Usage in QML becomes:

WinTaskbarButton {
    progress.value: 50
    progress.visible: true
}

Change-Id: I5cd4b2d99fc110709e056b3d6c5f0c89b03735e3
Reviewed-by: Ivan Vizir <define-true-false@yandex.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>