Skeleton locking

For avatar systems, multiple users may work on the same skeleton, adding new animations. Every user gets their own copy of the master skeleton and may only add new animations, but may not add new bones/slots/attachments or change the base configuration of the skeleton. New animations then get merged back into the master skeleton.

To facilitate this workflow, the editor should allow to lock or freeze a skeleton so only modifications that can be merged back into the master skeleton can be performed. This prevents accidental changes that prohibit merging changes back into the master skeleton.