[easytag] Improve MP4 album artist support with newer TagLib

[David King <davidk src gnome org>]

commit 3b4766f81173eb3bf3b416295ae4984572d8e06e
Author: David King <amigadave amigadave com>
Date:   Wed Apr 27 17:22:43 2016 +0200

    Improve MP4 album artist support with newer TagLib
    
    TagLib 1.10 and above supports the album artist field directly, without
    the need to manually retrive the atom. Use this support if building
    against a suitable TagLib version.

 src/tags/mp4_tag.cc |   22 +++++++++++++++++-----
 1 files changed, 17 insertions(+), 5 deletions(-)
---
diff --git a/src/tags/mp4_tag.cc b/src/tags/mp4_tag.cc
index 2bdabb5..d24695c 100644
--- a/src/tags/mp4_tag.cc
+++ b/src/tags/mp4_tag.cc
@@ -224,14 +224,21 @@ mp4tag_read_file_tag (GFile *file,
 
     const TagLib::MP4::ItemListMap &extra_items = tag->itemListMap ();
 
-    /****************
-     * Album Artist *
-     ****************/
+    /* Album Artist */
+#if (TAGLIB_MAJOR_VERSION == 1) && (TAGLIB_MINOR_VERSION < 10)
+    /* No "ALBUMARTIST" support in TagLib until 1.10; use atom directly. */
     if (extra_items.contains ("aART"))
     {
         const TagLib::MP4::Item album_artists = extra_items["aART"];
         FileTag->album_artist = g_strdup (album_artists.toStringList ().front ().toCString (true));
     }
+#else
+    if (extra_tag.contains ("ALBUMARTIST"))
+    {
+        const TagLib::StringList album_artists = extra_tag["ALBUMARTIST"];
+        FileTag->album_artist = g_strdup (album_artists.front ().toCString (true));
+    }
+#endif
 
     /***********
      * Picture *
@@ -433,17 +440,22 @@ mp4tag_write_file_tag (const ET_File *ETFile,
     TagLib::MP4::ItemListMap &extra_items = tag->itemListMap ();
 
     /* Album artist. */
-    /* FIXME: No "ALBUMARTIST" support in TagLib until 1.10, use atom
-     * directly. */
     if (!et_str_empty (FileTag->album_artist))
     {
         TagLib::String string (FileTag->album_artist, TagLib::String::UTF8);
+#if (TAGLIB_MAJOR_VERSION == 1) && (TAGLIB_MINOR_VERSION < 10)
+        /* No "ALBUMARTIST" support in TagLib until 1.10; use atom directly. */
         extra_items.insert ("aART", TagLib::MP4::Item (string));
+#else
+        fields.insert ("ALBUMARTIST", string);
+#endif
     }
+#if (TAGLIB_MAJOR_VERSION == 1) && (TAGLIB_MINOR_VERSION < 10)
     else
     {
         extra_items.erase ("aART");
     }
+#endif
 
     /***********
      * Picture *