gameplay mod: water that is covered by constructions doesn't freeze

[myk002]

allow player to build water shelters and make the sheltered water not freeze in winter

we can do this by periodically scanning the map and identifying all tiles that are skyview+inside that have a solid construction (everything other than fortification and stairs?) above them. Mark such tiles as underground+inside.

conversely, we'll also have to identify tiles that should be marked back to skyview when constructions are deconstructed

probably want to be a plugin due to map tile iteration required for periodic sweep (though most transitions can be done in an EventManager callback for new/removed construction)