Artist and library playlists are allocated manually, so there should be
generic functions that both can use to get a playlist pointer and free
it when we're done.
I also add a callback for telling the UI when new playlists have been
allocated. This isn't needed for Library playlists, but this is the
only way the UI can know about new Artists.
Signed-off-by: Anna Schumaker <Anna@NoWheyCreamery.com>