Mapgen

The mapgen is in a pretty good state, but needs to be finished off.

Structures need to be added. How? Schems? Lua code? tbd. For structures, we need to add more decorative blocks.

The sea coast needs to be finished. At the moment, it drops down too rapidly.

The patch for perfecting hight transitions needs to be pushed.

More plants (farming, athelas, etc.) need to be added to the mapgen.