Button's text should use SystemPalette.buttonText

In themes with inversed colors text and buttonText colors may differ,
and Button as QButton's analog should use the correct one.

Change-Id: I101e0c463faf5f99df0282486aef0d3411b41156
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>