Support random mixes without creating a radio station

This implements random mixes for artists, albums, tracks, and genres.
It is also possible to retreive artist-only mixes which is identified
by a special seedType. Artist-related mixes use the seedType 5,
whereas artist-only mixes use seedType 7.

During the development of this I identifed the seedTypes as follows:
1: My Music Track Related Mix
2: All Access Track Related Mix
3: Artist Related Mix
4: Album Related Mix
5: Genre Mix
6: I'm feeling lucky radio
7: Artist-only Mix

It seems that there is also seedType 8, but I don't know what this is.

Splitting up all kinds of mixes into separate client methods seemed to
be more intuitive than using just one. What do you think?