In the multithreaded version, the "directory done" state was written before
other progress states, causing it to be overwritten. This was because I had
stupidly put the "done" message in the initial generator, which of course
finishes before the entire pipeline finished. This manifested as two problems:
the tagger would always want to "resume" even when it had finished the last
time; "aBort"ing the process would not cause the next run to resume.
This makes the apply_choices coroutine run even for albums that are skipped or
still in the library. This (along with making things more predictable) lets the
apply_choices stage write the progress value as albums are retired even if they
are skipped.
As part of this, the BaseLibrary class was also adapted to include a notion of
albums. This is reflected by the new BaseAlbum class, which the Album class
(formerly _AlbumInfo) completely replaces in the concrete Library. The BaseAlbum
class just fetches metadata from the underlying items.
Previously, importing without autotagging just imported a bunch of Items. Now,
like the autotagging version, "import -A" creates albums based on the directory
hierarchy. The effect is exactly as if the user chose "use as-is" every time in
the interactive procedure. One side effect is that "import -A" can now only take
directories, where previously it could take single items on the command line. We
need a new solution for this kind of import in the future.
