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

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

src/command/DatabaseCommands.cxx

@@ -124,7 +124,8 @@ handle_searchaddpl(Client &client, int argc, char *argv[])
 	if (db == nullptr)
 		return print_error(client, error);
 
-	return search_add_to_playlist(*db, "", playlist, &filter, error)
+	return search_add_to_playlist(*db, *client.GetStorage(),
+				      "", playlist, &filter, error)
 		? CommandResult::OK
 		: print_error(client, error);
 }

src/command/PlaylistCommands.cxx

@@ -196,7 +196,8 @@ handle_playlistadd(Client &client, gcc_unused int argc, char *argv[])
 		if (db == nullptr)
 			return print_error(client, error);
 
-		success = search_add_to_playlist(*db, uri, playlist, nullptr,
+		success = search_add_to_playlist(*db, *client.GetStorage(),
+						 uri, playlist, nullptr,
 						 error);
 #else
 		success = false;