Fix settable style hints.

Introduce QStyleHintsPrivate and introduce internal
setters called by QApplication.

Task-number: QTBUG-33991
Change-Id: Id61f8b1e2b5c9cfd7b4713aaded66e93e6f63719
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>
Reviewed-by: Andy Shaw <andy.shaw@digia.com>