Label should use SystemPalette.windowText

In themes with inversed colors text and windowText colors may differ,
and Label as QLabel's analog should use the correct one

Task-number: QTBUG-37468
Change-Id: Ie4f574e782b1c9274d86f45433c36ecab2989b97
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>