qmlscene: if top-level is a QQuickWindow, set incubation controller

qmlscene: if top-level is a QQuickWindow, set incubation controller

The necessity for this is documented in the
QQuickWindow::incubationController() accessor.  Bug 31203 is about
writing a custom main() with a QQmlApplicationEngine, not about
qmlscene, but the same workaround is necessary for both.  Hopefully
soon we find a way to make it unnecessary, but for now it is.
This fix also doesn't take care of the QtObject { Window { } }
use case.

Task-number: QTBUG-31203
Change-Id: Ic9a5e4a23ac918593138dc9cab3de6cd36453eaf
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>
Reviewed-by: Alan Alpert <aalpert@blackberry.com>