This rolls back the places in #2094 where `six.text_type(s)` was used where
`s` might be a bytestring. We almost never want that conversion, because it
uses the default encoding (almost always ASCII). The need for it indicates a
problem somewhere else, where bytes were produced when we were expecting
unicode strings.
If `medium_index` is missing, as it is from the Beatport source, the display
would show "None" instead of the global index (a fallback). This is half of
the problem in #2085.
* Fix docstring and `plugins.rst` regarding how the PromptChoices that
use the same short letter are handled (they are discarded instead of
raising an Exception).
* Rename the importer argument and related variables to make it more
generic, as the feature should be independent of the backend used and
not restricted to MusicBrainz.
* Update documentation and docstrings accordingly.
* Add changelog entry.
* Style cleanup and fixes for the "--musicbrainzid" import argument.
* Allow the input of several IDs (separated by spaces) on the "enter Id"
importer prompt.
* Add basic documentation.
* Modify the "--musicbrainzid" argument to the importer so multiple IDs can be
specified by the user instead of a single one.
* Revise autotag.match.tag_album and autotag.match.tag_item signature to expect
a list of IDs (search_ids) instead of a single one (search_id), and add logic
for handling and returning multiple matches for those IDs.
* Update calls to those functions in other parts of the code.
* Add '-m', '--musicbrainzid' option to the import command, allowing the user
to specify a single MusicBrainz ID to be used for the candidate lookup instead
of trying to find suitable candidates.
* Modify lookup_candidates() of ImportTask and SingletonImportTask to use the
musicbrainz id if present.
* Simplify PromptChoice so "plugin" and "id" fields are removed, updating the
loops and the rest of the code to reflect this change.
* Solve short letter conflicts by keeping one of the choices and removing the
rest, instead of by raising an Exception.
* Misc cleanups as suggested on #1758 discussion.
