Shared content proxy

Not sure whether this could have negative side effects for other use cases, but it allows me to reference `self.parent.content` in a `Content` base class, with zero additional queries, to implement methods like `is_first_in_region`, `is_first_of_type`, `last_in_group`, `earlier_siblings`, etc. Could easily implement as a custom `ContentProxy` base class but wondered if this functionality, or a variant thereof, may be widely useful.