Make terrain modifiable

[Trilarion]

Select new terrain, ask if changes to/from water and modify internal scenario structures accordingly, then redraw the map.

[amtyurin]

Terrain modifications are working now in editor mode

[Trilarion]

Didn't work for me right now, needed to change a line in c4967a6. Also there are still things to do right if you change solid terrain to water, also the boundaries of a province would need to be changed. They aren't yet.