NOTES

Main stuff to look for when working with libspotify :
 - it's not because we have created a track/album/playlist/... that it is loaded
 - it's not because we have a sucessfully loaded track that we can play it

 For this simple player, I should event not have though about permitting multiple
 uris as arguments. The code works as follow :
  - put all uris in state->urisToPlay
  - for each uri, try to load all the tracks of its content, and add them to
  	tracklist
  - when effectivly adding a track to tracklist, checking that it is available,
    otherwise forget about it
  - when tracklist has been populated, forget everything about uris, playlist,
  albums, and only work with the tracks loaded in tracklist.