use private __maximumDimension in DefaultDialogWrapper

Correction to 35589b1f5a4e86856c7024bbfb067e0835c703f6

Change-Id: Ic0f89fe138be0f37042538ecce75ab31d1bc3ac6
Reviewed-by: J-P Nurmi <jpnurmi@theqtcompany.com>
Reviewed-by: Liang Qi <liang.qi@theqtcompany.com>
CustomDialogs example: add Apply, Reset and Restore Defaults buttons

A custom dialog might have a use for these.

Change-Id: Ibef05dd63d65476e1225477d2e7ed9d24fbba9ed
Reviewed-by: Liang Qi <liang.qi@theqtcompany.com>