{queue,song}_print: print relative paths if possible

If a song with an absolute path points inside the music directory,
print only the relative part.  This happens when partial songs from a
playlist file were loaded.

src/mapper.h

@@ -41,6 +41,14 @@ void mapper_finish(void);
 bool
 mapper_has_music_directory(void);
 
+/**
+ * If the specified absolute path points inside the music directory,
+ * this function converts it to a relative path.  If not, it returns
+ * the unmodified string pointer.
+ */
+const char *
+map_to_relative_path(const char *path_utf8);
+
 /**
  * Determines the absolute file system path of a relative URI.  This
  * is basically done by converting the URI to the file system charset