Make TextFieldStyle and SpinBoxStyle respect font size

This makes both the controls respect a custom font size.
Before this patch these controls would only support the
global application font.

Task-number: QTBUG-34750
Change-Id: I200a14adba5b75088ab54ba691430330d9226ada
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>