Adds a playlist folder type, it is needed for sorting since some clients (eg Retune)

require that playlist folders are sent before their content. Playlist folders should,
however, be sent after the base playlists, so the numbering is changed. At the same
time makes the value of the smart playlist type a bit less hardcoded.

src/db.h

@@ -165,10 +165,12 @@ struct media_file_info {
 
 #define mfi_offsetof(field) offsetof(struct media_file_info, field)
 
+/* PL_SMART value must be in sync with type value in Q_PL* in db.c */
 enum pl_type {
   PL_PLAIN = 0,
-  PL_SMART,
-  PL_MAX
+  PL_FOLDER = 1,
+  PL_SMART = 2,
+  PL_MAX,
 };
 
 struct playlist_info {