PictureFlow: id3 functions refactoring

- Move multiple-id3 functionality into lib
- Extract retrieve_id3 function

In preparation for using these functions
from the Properties plugin.

Change-Id: I7252581340680e32df763f1f4b41646a055fe19e

apps/plugins/lib/mul_id3.h

@@ -0,0 +1,28 @@
+/***************************************************************************
+ *             __________               __   ___.
+ *   Open      \______   \ ____   ____ |  | _\_ |__   _______  ___
+ *   Source     |       _//  _ \_/ ___\|  |/ /| __ \ /  _ \  \/  /
+ *   Jukebox    |    |   (  <_> )  \___|    < | \_\ (  <_> > <  <
+ *   Firmware   |____|_  /\____/ \___  >__|_ \|___  /\____/__/\_ \
+ *                     \/            \/     \/    \/            \/
+ * $Id$
+ *
+ * Copyright (C) 2023 Christian Soffke
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ ****************************************************************************/
+#ifndef MUL_ID3_H
+#define MUL_ID3_H
+
+void init_mul_id3(void);
+void collect_id3(struct mp3entry *id3, bool is_first_track);
+void write_id3_mul_tracks(struct mp3entry *id3);
+
+#endif /* MUL_ID3_H */