Terrain editing: Automatically place beach when placing water about openage

[zuntrax]

We should also consider the opposite case: Placing e.g. grass tiles in water or beach.

[mic-e]

Good catch. Whenever a tile is edited, the surrounding tiles should be checked for water-land borders, and replaced by beach if one is found. Harder mode: Beach sometimes needs to be auto-removed.

[TheJJ]

This should be available as an opt-in or out option, for testing purposes one might want to place exactly the terrain tiles wanted, no implicit stuff should happen then.

[franciscod]

Maybe we could have clickable checkboxes on the f3 debug menu for all those things :)

[hackathi]

I think this should be a requirement for /doc/editor/terrain.md.