playlists: Rename StaticPlaylists to SmartPlaylists
Favorites, RecentlyAdded, RecentlyPlayed, NeverPlayed and MostPlayed playlists are supposed to be call SmartPlaylists in GNOME Music terminology.
In summary, it exists two types of playlists:
- SmartPlaylists: playlists are auto generated
- user playlists: playlists created by the user