All models have been added a column to store the DiscoveryStatus for the
song, and this column is passed on set_playlist(), so the Player object
does take care of updating the status after the song failed playing, or
discovery failed on it.

This makes sure we react properly to player/discovery failures for the
current and next songs, and only when it's needed so. Discovery is a
very expensive operation, and this allows us to stop doing it for every
song at the time of adding these to a model/view, which means lots of
I/O and CPU time just to know whether songs can be played, even it they
might not eventually.