populate_playlists was used to retrieve all the playlists (smart
playlists and playlists created by the user). The only consumer of
this funtion was MPRIS which now uses the playlists queried by the
Playlists class.

Playlists class uses a combination of populate_user_playlists (to
retrieve user playlists) and direct tracker calls to get all the
playlists. The tracker calls are necessary because the smart playlists
need to be created if they don't already exist.