Water height

This PR adds support for reading the sea level height from a generated world instead of using a fixed height of 32.

* A method `int getSeaLevel();` was added to `World`. I also thought about putting it in `WorldGenerator` or `WorldProvider`, but `World` seems to be the most sensible choice to me.
* It does not directly use the facet info, because that is defined per region and could change. 
* I renamed SeaLevel to WaterLevel to make it more clear that this not a constant.

Closes #1371