Improve persistence using object id in scene file

[rstecca]

• https://answers.unity.com/questions/1238307/how-can-i-match-a-gameobject-to-its-id-in-the-unit.html
• https://forum.unity.com/threads/how-to-get-the-local-identifier-in-file-for-scene-objects.265686/
• https://docs.unity3d.com/ScriptReference/AssetDatabase.TryGetGUIDAndLocalFileIdentifier.html
  https://answers.unity.com/questions/1249093/need-a-persistent-unique-id-for-gameobjects.html