Tile borders & bug fixes

* TileBorders Generator introduced.
It produces borders between biomes and floors. It is still far from
perfect and has many errors. The main issue is that saving the borders
properly in the OWF format file takes up 6 bytes on each tile and that
is a very big amount.
* Rivers will no longer broaden on tiles that are higher.
* Biome borders will only be applied on the same floor.
* The water level is no longer a separate floor.
* The top floor is no longer lowered to the lowest one.

Hope I didn't forget anything :)