Change the way client's collections are managed (list of tracks,
 albums, artists, etc.)

- Fetch from the server a complete list of tracks and populate
  AppStorage.get('tracks') with it.
 - Instanciate Album models from that list of tracks.
 - Instanciate Artist models from that list of tracks.
Add two entries in the API

- '/list_songs', return a list of every songs in the mpd db
- '/list_playlists', return a list of every playlists