* Add tests for applying added prefab instances to prefabs
* Fix nested prefab instances getting wiped when applying changes
Prefab update serialization only kept instance data for nested roots sitting directly under the instance root, deeper ones (under group objects) got their mappings wiped in the prefab cache scene where they can't be rebuilt.
Also only convert children to nested after applying, flagging the applied root itself made it get silently adopted when dragged into another instance later.
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