Avoids the ugly contorting of the data model from arbitrary directory
structure to artist-album-track. This should make the API requests
actually match the original data model.

- I have only tested with Navidrome. Since Navidrome uses identical IDs
  for the fake hierarchy and the tags, the transition is seamless.
- I haven't tested Subsonic. Since it uses different IDs for directories
  vs. tags, it's likely this won't be so smooth. Recreate your server.
  (Could this be handled better?)
- Nor have I tested other implementations.
- Podcasts may be busted. The podcast functionality is really bitrotted
  and not well tested regardless.
- We could do real directory hierarchies (GH-173) correctly, with a data
  model actually modelling... directories.

The first element is subtly different for getAlbumList(2)

It gets a bit wobbly looking if there's some irregular entries and
albums get cleared out via item not found errors popping up, but it does
make migration surprisingly realistic.

The podcast stuff is still using one of them though...

Reset IDs, names, and paths when possible. Better handle object
ownership when possible.

It returned application/x-download, which would confuse the logic. It
shouldn't really be relying on file extensions anyways, but the content
of the files. (TagLib when?)

Not strictly needed for ID3 work, can be cherry picked back to master

Could be made common...

...since they'll always be invalid. Will clear up entries in a Subsonic
directory index to ID3 transition, better than empty set would. Also
purges the name-only entries too!

If we do directory support again, we should handle it correctly.

Maybe we can clean up the request types?

Some of it can't be removed until podcasts, but it seems podcasts are
going to be really crumbly until that...

This was never working in the first place. The podcast menu item didn't
switch, and items were duplicating. And the stream URL was for the wrong
ID type. Things now work, so it's at least better than it used to be.

Still some grungy stuff - there's no way to manipulate the podcasts, for
instance. And the automatic download stuff has some curious ideas about
metadata? Lots of UI thoughts for anywhere where podcasts intersect w/
noormal tracks.

(Or maybe we can remove it?)

I think it's because in Subsonic, the album objects can't be rated, but
directories can be. Weh.

When transitioning to the new schema, you'll likely get albums that
don't exist, though we try to update as soon as possible when the artist
album list is pulled. My guess is a race condition between getting the
album list from the server and the cached albums from Core Data.

The easiest workaround is to just not show this message.

There were some uh, nasty playlists on demo server that got cleaned up
remotely, but not locally