Correct positioning for popups in QQuickWidget

Handle the case where the QQuickWindow is hidden and the content
is rendered somewhere else using QQuickRenderControl.

Task-number: QTBUG-38116
Change-Id: I51fb9001c307f53bf7d724860b2a66287982724e
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>