XWiki-9282 : Add a way to restrict access to some classes

The idea is to be able to restrict some classes to users with PR or admin right. It means that only users with this right would be able to create, edit or delete objects from this class.
This should be useful from a security point of view, since some objects can represent a danger (JavascriptSkinExtension for a start as they can be used to introduce XSS), and I believe this improvement could also be helpful for introducing SignedScripts.
The next step is to use it for restricting JavascriptSkinExtension to authorized users.