Export the QmlEngine used by Qt3D::QuickWindow

We can now access its root context and set properties on it,
create components, etc.

Change-Id: I9ee625c2e06019a4b3a3c3aa2719e3c1dc8b8dc0
Reviewed-by: Paul Lemire <paul.lemire@kdab.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Skeleton example for loading (big) models from the command line

Unfortunately the Instantiator doesn't work yet (that is,
dynamically updating the scene has no effect)

Change-Id: I940c192440c9fb65e94e3b20a474022d2164bcda
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>