Refactor QWinTaskbarProgress out of QWinTaskbarButton

Change-Id: I8b95d7804d9070c8e93993ad6acf5c22d964880d
Reviewed-by: Ivan Vizir <define-true-false@yandex.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Make QWinThumbnailToolBar::clear() a slot

Change-Id: I65b08d553bc06e4c378cc76977b719cffd8e48b5
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>