Fixed: MUSINFO was not multiplayer-aware

- Move MUSINFO change request out of FLevelLocals and into player_t. This
  allows the MusicChanger actors to change music for each player
  independantly. This is similar to PrBoom+, which switches depending on
  the displayplayer. The difference being, we don't actually track the
  music other players are listening to. (Which might not be a bad idea to
  implement at some point.)
- Moved a few fields in player_t for better packing.

src/g_level.cpp

@@ -1457,7 +1457,9 @@ void G_SerializeLevel (FArchive &arc, bool hubLoad)
 
 	if (SaveVersion >= 3313)
 	{
-		arc << level.nextmusic;
+		// This is a player property now
+		int nextmusic;
+		arc << nextmusic;
 	}
 
 	// Hub transitions must keep the current total time