[ModelCompatibility] Revert model version to 2.6.0

* No need to do this, as we shall see in the next commit
[ModelCompatibility] Improve compatibility checking

* Better checking
[ModelCompatibility] Add Unit Tests
[Unit Tests] Close Editors asynchronously
[Copy/Paste] Child objects are now copied to the correct relative positions

* I am the brain king!
Merge branch 'model_compat'