Added functionality to shuffle albums (exposed via the sort submenu)

For this a new class "AlbumList" was added, which fetches all album titles from mpd and offers the possibility to iterate through all albums and randomly sort all albums or play a random album.
I'm not sure this is the right way to go, maybe this could be abstracted a little bit more. Feedback would be great!
Also the whole concept of an "album" in a playlist is just consecutive entries in the playlist with a given album title, and therefore the whole code depends on the playlist being sorted by album title. This is good enough for me, but maybe this needs more refinement.