Button's text should use SystemPalette.buttonText
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 <9cea9d70b08f8210134ec5959114f72bad51ed1d@digia.com>

mizukusak/qtquickcontrols,mizukusak/qtquickcontrols,mizukusak/qtquickcontrols,drgvond/qtquickcontrols,drgvond/qtquickcontrols,drgvond/qtquickcontrols