Add support for smart playlists

[richienyhus]

Smart playlists are dynamic playlists that retrieves all songs matching a particular criteria by searching for it in the metadata.

• a playlist for most played tracks,
• a playlist for most played tracks of last year,
• playlist for tracks you have never played
• playlists for different genres,
• playlists for different decades,
• a playlist for highest rated tracks
• a playlist for non rated tracks
• recently played playlist (played in the last fortnight)
• not recently played playlist (not played in the last fortnight)