Improved multi-disc album detection.

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

- Don't only expect each disc to be in a subdirectory of a common parent
  directory, with all siblings belonging to the same release. Also match
  any consecutive siblings (even when the parent contains other albums)
  that are named with the same prefix and multi-disc marker.

- The `albums_in_dir(path)` function now always yields a list of paths
  along with 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 "; " by default. This can be changed with the
  `separator` argument.

- The `sorted_walk()` function now does a case insensitive sort on
  directories, but still returns case sensitive results. This allows
  better multi-disc album detection.

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