Add (not)inplaylist operator to smart playlists (#1884)

Closes #1417 

A smart playlist can use the playlist id for filtering. This can be
used to create combined playlists or to filter multiple playlists.

To filter by a playlist id, a subquery is created that will match the
media ids with the playlists within the playlist_tracks table.

Signed-off-by: flyingOwl <ofenfisch@googlemail.com>

model/criteria/json.go

@@ -66,6 +66,10 @@ func unmarshalExpression(opName string, rawValue json.RawMessage) Expression {
 		return InTheLast(m)
 	case "notinthelast":
 		return NotInTheLast(m)
+	case "inplaylist":
+		return InPlaylist(m)
+	case "notinplaylist":
+		return NotInPlaylist(m)
 	}
 	return nil
 }