API change: extend PermissionDelegate to allow checking whether a user can create within a target

Introduce the following:

canCreateInTarget(User user, Class type)

If a plugin author declares a new entity type called Foo.class and Foo can contain Attachment's, they will be able to implement:

FooPermissionDelegate.canCreateInTarget(user, Attachment.class)

To define what user is able to create instances of Attachment.class within instances of Foo.class.