Compounding Smartness

Banshee's smart playlists can now be derived from other smart playlists. Since each one allows you to AND or OR a set of conditions, by compounding them you can create the full range of boolean queries. You can create playlists that match against songs' metadata (including rating, when last played, number of plays, etc) and whether a song is (or isn't) in a normal or smart playlist, and then cap the playlist at some number of songs, minutes/hours, or MB, with preference to higher ratings, or whatever you want. I also added a predefined smart playlist for "Unheard Podcasts".