Make Status extend StatusValue and start FileBackend update

Make Status extend StatusValue and start FileBackend update

* This avoids code duplication.
* Callers can safely start type-hinting StatusValue as well.
* Also moved the wrap() logic out of Status::__construct(), which
  only wrap() was (and should have been) using. Use "static" as
  well, so subclass behave properly.
* The docs and type-hints in /filebackend are updated as an example.
* A migration pattern is to inject a StatusValue wrapper
  into the backend and use it on all returned statuses, so MediaWiki
  will still get Status for message methods.

Change-Id: Iff9255f34870ea6b0c4b91f6ddc69eea95186aba