Improved multi-disc release detection.

- Remove "part", "volume", "vol." multi-disc markers. These are often
  part of release titles, and not necessarily indicative of a multi-disc
  release. Only look for "CD X" and "disc X", ignoring white space and
  other non-word characters.

- Don't expect each disc to be in a subdirectory of a common parent
  directory, with all siblings belonging to the same release. Match any
  consecutive siblings (even when the parent contains other releases)
  that are named identically, except for the disc number.

- The `albums_in_dir(path)` function now always yields a list of paths
  for each list of items. `ItemTask.path` is now always a list of paths.

- The `displayable_path(path)` function now accepts a list of paths, and
  will join them with "; "

- The `art_for_album()` function now takes a list of paths as its second
  argument, instead of a single path.
Fix multi-disc album tests.

- Allow "_" as non-word character after multi-disc marker.
- Don't yield empty multi-disc albums.