Re: [PATCH] Add a source for beets library databases

I've pushed an updated version of the patch to a new "beets" branch of
the EMMS repo.

This version:
- ensures at least a name and a type is available for each added track,
- inhibits info functions when adding from the beets database,
- allows interactive filtering by distinct value(s) of column(s) when adding,
- sorts tracks to be added before inserting them into the playlist,
- allow customizing the columns to sort by, their order and the sort type,
- only gets data from a predefined set of columns,
- attempts to fix the data format of some columns and their names,
- adds various small improvements and optimizations to the code.

This is basically feature complete IMO.
I could add an info function which gets metadata from the beets DB if a
file has been added without using the beets source commands but is
actually registered in the DB, however I'm not sure how useful this
really is.