Add ability to directly place QQuickItems in 3D scene without going through Texture component

It should be possible to render 2D Quick content in 3D scenes using the geometry and material's provided by Qt Quick, though this may be difficult in Qt 5.  We should start to investigate the feasibility of doing this now though, and add a convenience path if necessary.  So for example given this scene:

Node {
    Camera {
        z: -600
    }
    Light {

    }

    Node {
        x: 100
        y: 100
        rotation: Qt.vector3d(45, 0, 0)

        Rectangle {
            height: 100
            width: 100
            color: "blue
        }
    }
}

It should be possible to recognize that there is a 2D component in the scene, and render that as an item in 3D space.  Either we use the state directly backed into the item itself, or we create a container for it that implicitly creates geometry and a texture that transparently looks like the item was "embedded" into the scene.  Either way I suspect users will want to use the API in this manner, even if we have to do magic under the hood to make it possible.