Sea on ice

[Soyweiser]

Found a few bugs with this:
- Units can still retreat into the water or out of the water. Ignoring the ice.
- Cloud trapezing into the ice still works. (Not out, but mages cannot cast cloud trapeze underwater anyway).
- Flying behaves oddly when underwater. If a flyer starts in an underwater province. It can still fly over normal provinces when the sea of ice is up. As long as it lands in a water province.