Allow locking/forking/reverting case-types

There are a few different use-cases for file-based CiviCase XML, and a key differentiator is whether the site-admin should be allowed to manipulate the definition through the UI. Web-based editing is handy for local-customizations – but problematic for applying updates or deep integrations (with custom-code).

This ticket addresses the issue by adding a new XML tag, <forkable>. By default, CiviCase XML files are considered forkable, so the admin may edit the definition. However, if the developer declares "<forkable>0</forkable>", then the definition cannot be changed.

If a case-type has been forked, we should mark the case-type in bold and add the "Revert" action.

Note: The <forkable> property applies to creation of new forks. If you have already forked a case-type, then it won't automatically revert. Reverts must be done manually.