Cocoa: Make sure that resizeEvent is invoked after calling resize

The QWindow::resizeEvent documentation states that resizeEvent
is invoked after the windowing system has acknowledged a
setGeometry() or resize() request.

The Cocoa plugin however did set the platform window geometry
immediately so that the qnsview's updateGeometry returned too
early.

Task-number: QTBUG-32706
Change-Id: I1f359ab368833d174ab6740f4467b0848c290f13
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>