Hide compilation artists and fix album track ordering (#15)

This resolves #15 (closed), or at least the two issues mentioned in it:

• Viewing an album in search results (or through any other method that calls library.lookup) returns tracks in reverse order. Sorting album tracks by position fixes this. (0cb540ef)
• Browsing artists can get very cluttered if a library has a lot of distinct per-track credits, such as collaborations. Funkwhale has an "Exclude Compilation Artists" option on the artists page. Adding a similar option to the extension to filter artists by has_albums: true fixes this and makes browsing artists return the same list as the Funkwhale artists page with the respective setting. I've set the default to true to match the behavior in Funkwhale. Searching for an artist still searches all artists. (bef84395, 9b00b87d)

#15 (closed) also mentions adding support for playlists, but I haven't included anything related to that because it was suggested as a workaround for the above issues. Playlist support would still be a nice feature to have though.