Lake Height GUI

[kierongreen]

With speed up to lake code it's more feasible to tweak some parameters for them. This patch adds a gui to set maximum lake height above sea level - 0 means no lakes will be generated, 8 was default before.

Haven't committed this to trunk as will need an increment to settings version and don't know whether want to wait for a few changes before we do that. Therefore patch still writes bool for now.