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/db/update/UpdateSong.cxx

@@ -50,6 +50,7 @@ try {
 		}
 
 		new_song->mark = true;
+		new_song->added = std::chrono::system_clock::now();
 
 		{
 			const ScopeDatabaseLock protect;