QPushButton: draw label using the stylesheet's font.

The RenderRule for the stylesheet has the correct font information, it
just wasn't being set on the painter that draws the label.

This is a backport of the Qt 5 commit:
9ad768fd1999780df18551151b9d9fa9c354131c.

Task-number: QTBUG-8990
Change-Id: I4857d6e17b62980782ca7bfdfd0a571a9950f635
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>