Refactor level collections

[lahm86]

Refactor each of the TRLevelReader.Model classes to use lists for the various collections instead of arrays. ~~Retain the related NumXYZ properties, but remove the setters.~~

e.g.

public ushort NumRooms { get; set; }
public TRRoom[] Rooms { get; set; }

will become

public List<TRRoom> Rooms { get; set; }

and callers can just use the built-in count to get the relevant numbers.