Schema can still be changed even when not checked out

[Geert Bellekens]

Details:
If you put a schema composer artifact in a controlled package the schema itself is not rendered read-only. It can still be changed even without checking out the package.
The same problem with the locking mechanism of user security.

Steps to Reproduce:
- Create a schema composer artifact in a controlled package
- Check in the package
- Try to update the schema in the schema composer
- Notice that you can still change the schema although it should be read-only.

Reported

Geert