Enable/prove multiple SceneManagers can co-exist

The viewer has a Qt 3D environment, a UipDocument, and a SceneManager. The latter builds and runs the Qt 3D scene.

With small enhancements this can be extended to be able to have 2 UipDocument and SceneManager instances, where the framegraph subtree from the second SceneManager targets a texture. (i.e. one onscreen + one or more offscreen 3DS presentations, all co-existing in the same Qt 3D environment.

Provide the necessary plumbing and prove that it works.
(tag entities, merge framegraphs, avoid potential issues with cached objects parented to something in one scene's entity tree, etc.)

Pre-req for further subpresentation work.