HaikuDepot: Minor style cleanup.
HaikuDepot: added "available" filter.

* That's the only one I would ever want to see (in most cases).
* I must admit that I don't really understand why the lists are created
  and updated only for filtering, instead of just testing against the
  package state.
* However, I also have no idea, why the NotContainedInFilter does not
  work. I left it in because someone might see what I didn't, and maybe,
  if the first confusion is cleared, it might even find some use.