Improve sorting in library view

[vvnull]

From the library, if one drills through the Genres/Artists/Albums menus (so not Files or Playlists), the tracks within an album are sorted by track title, rather than the expected track number.

Furthermore (though less important), sorting albums within an artist by the year tag would be nice.

[yktoo]

This is problematic because of the limits imposed by the MPD API.

MPD's list command returns a list of simple strings (like album or track names), and not proper objects carrying metadata. Which means sorting would require an additional query (or queries).

It's even more problematic when it comes to albums since the year tag is stored in each track's file, thus can potentially have different values for the same album.