Inspector v2: Add scene node to scene explorer

[ryantrem]

Two things in this PR:

1. Add a top level scene node to scene explorer.
2. Replace the scene explorer section "watch" function with just an array of observables. This is what I originally wanted to do, but it was hard to do because of the lack of covariance in Observable. It's now easy to do with the covariant IReadonlyObservable.

[bjsplat]

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To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

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[bjsplat]

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[bjsplat]

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