Add "added" timestamp to song database

- added is set to current time, if a new song is added to the database.
- GetAdded falls back to mtime.

Code for proxy plugin is missing, this needs a patch for libmpdclient.

closes #378

src/playlist/PlaylistSong.cxx

@@ -25,6 +25,7 @@ merge_song_metadata(DetachedSong &add, const DetachedSong &base) noexcept
 	}
 
 	add.SetLastModified(base.GetLastModified());
+	add.SetAdded(base.GetAdded());
 
 	if (add.GetStartTime().IsZero()) {
 		add.SetStartTime(base.GetStartTime());