When using a QQuickWidget, the QQuickWindow is hidden and the contents
are displayed in another window. Some components need to query the actual
window, e.g. when positioning a popup.

Task-number: QTBUG-38116
Change-Id: I34452be2179ccc9e216e4d89264dc700e0cf42a0
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>