Unwind the picker abstraction into smaller dialog packages.
While this increases repetition, it will make the playlist dialog
implementations much less shoddy.
Add a dialog for picking a genre from several choices.
This basically completes multi-genre support in Auxio, save more
internal reworks.
Note that it is extremely likely that the "Play from genre" setting
will be removed soon. This feature has made me realize that such does
not many any real sense, as genres are more semantically similar to
playlists than artists or albums. This implementation only exists to
make multi-artist support an easy plug-and-play operation.
Resolves#201
