Room interiors revisited

Back some time I wanted to detach room interiors from Room class, because I thought Room class was becoming too big. I was afraid of big classes, but now I have to fix that by merging interior creation back to Room class. I also had an idea to create room interiors in large multi-room houses, but now I realize I could just create multiple rooms with each having their room interior and connect those rooms to create houses.

I'm actually fixing terrain_map references, but this is a part of it, because room interiors (and rooms) use lots of creation stuff. This time I'm avoiding direct calls to Level's creation routines and use a member for Room which then calls Level's routine. It's one way to emphasize modularity and make future modifications to the source code easier.