stored_playlist: moved functions from playlist.c

The two functions clearStoredPlaylist() and addToStoredPlaylist()
don't belong into playlist.c.  clearStoredPlaylist() was a wrapper for
spl_clear(), and is converted into a CPP macro for now.

src/stored_playlist.h

@@ -33,12 +33,16 @@ spl_move_index(const char *utf8path, int src, int dest);
 enum playlist_result
 spl_clear(const char *utf8path);
 
+#define clearStoredPlaylist spl_clear
+
 enum playlist_result
 spl_remove_index(const char *utf8path, int pos);
 
 enum playlist_result
 spl_append_song(const char *utf8path, struct song *song);
 
+int addToStoredPlaylist(const char *file, const char *utf8file);
+
 enum playlist_result
 spl_rename(const char *utf8from, const char *utf8to);