Expose renderType to SpinBox and TextField

This is useful in case you want to create your own scaling
styled controls.

Task-number: QTBUG-33917

Change-Id: I0d641319a3106b8e0971d0aae69799bc1aff7f78
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Thorbjørn Lindeijer <bjorn@lindeijer.nl>