Add support for multiple scenes / cross scene references

[McManning]

The sample mech file I'm trying to stress test with has two mains scenes - one contains the actual mech geometry and the other contains a reference to that for composition:

While loading the composition scene into Coherence - the mesh data isn't transferred because it's not digging into that referenced collection for objects to load.

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This also poses an interesting question about organizing the output scene in Unity - and how much it should replicate Blender's scene(s). I want to say that collections themselves should also be transferred over as an empty GO object type.