Mapper: move map_song_detach() to db/DatabaseSong.cxx

Use Storage::MapUTF8() internally, don't use global variables.

src/db/DatabaseSong.hxx

@@ -22,10 +22,20 @@
 
 #include "Compiler.h"
 
+struct LightSong;
 class Database;
+class Storage;
 class DetachedSong;
 class Error;
 
+/**
+ * "Detach" the #Song object, i.e. convert it to a #DetachedSong
+ * instance.
+ */
+gcc_pure
+DetachedSong
+DatabaseDetachSong(const Storage &storage, const LightSong &song);
+
 /**
  * Look up a song in the database and convert it to a #DetachedSong
  * instance.  The caller is responsible for freeing it.
@@ -34,6 +44,7 @@ class Error;
  */
 gcc_malloc gcc_nonnull_all
 DetachedSong *
-DatabaseDetachSong(const Database &db, const char *uri, Error &error);
+DatabaseDetachSong(const Database &db, const Storage &storage, const char *uri,
+		   Error &error);
 
 #endif