SQL-based paging everywhere, and various other changes

[FlorentLM]

As disussed in this issue, here's the PR that implements SQL-based paging.

However it also includes pretty heavy rewrites of the response-construction logic (and thus, of all endpoints functions), along with other minor additions / changes.

Full list (I think?):

• Response construction is done via one unified dict-like structure and then converted into json or xml, which greatly simplifies adding new endpoints
• All queries that can be paginated are paginated via SQL queries
• Genres splitting when a song or album has a comma (or other delimiter)-separated list
• Made the cover art logic more robust and faster, also fallsback to sending a cover from coverarchive.org when none is found locally
• Added support for any m3u format including Beet's playlist and smartplaylist plugins' playlists (as well as Beetstream's own playlist_dir option of course)
• Added support for native ffmpeg if the user donesn't have the python module installed
• Added getAlbumInfo and getAlbumInfo2 endpoints
• Various naming and stylistic changes to avoid shadowing built-ins (like id, etc) and for consistency

Tested with the following clients:

• Symfonium
• Ultrasonic
• Tempo
• Youamp
• GoSONIC
• Substreamer

Gonna start implementing support for authentication in another branch and then will add the missing endpoints :)

Cheers!