change pl_type for plain and special playlists and upgrade db to v18

src/db.h

@@ -168,10 +168,10 @@ struct media_file_info {
 
 /* PL_SPECIAL value must be in sync with type value in Q_PL* in db.c */
 enum pl_type {
-  PL_PLAIN = 0,
+  PL_SPECIAL = 0,
   PL_FOLDER = 1,
-  PL_SPECIAL = 2,
-  PL_SMART = 3,
+  PL_SMART = 2,
+  PL_PLAIN = 3,
   PL_MAX,
 };