Display sizegrip on QMdiSubWindows, even on 10.7 and later

We don't have 4-edge window resizing, so we need QSizeGrip to
be functional again in this case.

Task-number: QTBUG-32228
Change-Id: Ib66bc662f8bf0b521427755570bc1cd65fb28446
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>