Library grouping: add "by album artist"

[kryger]

Currently there's no way to group releases into "album artist / album" as per the desktop client. A library with lots of compilations becomes virtually unusable due to the amount of single-entry artists.

[physk]

I have added a Pull Request for this as i wanted it as well :) #190

[physk]

@amuttsch this can be closed now as it's been merged #190 :)