View flutter layer tree in inspector

Breaking it off of #1655.

Actually visualizing a layer tree would be really useful. Our solution currently for improving paint performance is saying:

1. Hopefully default widgets mostly does the right thing
2. Otherwise, you might want to introduce repaint boundaries manually at the right place in the layer tree.

But without being able to see the layer tree, it's difficult to optimize.

It's also useful for figuring out why things are clipping if the clipping are happening implicitly inside some render object we're unfamiliar with.