Add riverbeds to map generator

[AaronVanGeffen]

Earlier this year, we moved the sea level settings to their own tab in the landscape generation window. This PR introduces the possibility to carve out river beds throughout the terrain map.

The resulting rivers aren't always the prettiest. Indeed, the code is a bit naive in places, and perhaps not the prettiest. Perhaps we can improve it a little over the course of the review process. Either way, I think it's a good start.