A common usecase appears to be to set variables in the rootContext
before loading a url in a QQuickWidget. We there need to ensure there
is a QmlEngine to set variables on when this is attempted.

Change-Id: I07aff2104313eeb3fab902ea3c6043c3c82c50f7
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>