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!

[FlorentLM]

Just realised the XML version is completely borked haha Please ignore the PR for now!

[FlorentLM]

Oooookay I think we're good to go now - XML version seems to be fine in all the endpoints I could test (using Subtracks on Android).

I merged the dev branch into main so it also includes more stuff:

• Almost all the fields possible in a Child response
• Defining types to add things like play count, starred / liked, last played, to beet's database.
• Avoiding setting lastModified to every time it's queried - hopefully speeding up some refreshes if nothing changed in the db
• Various change of naming style

I realise this all became a lot of commits for yous to check though, sorry about that...