[ATLAS] Object placement constraints

[historic_bruno]

I think this would be a good addition to Atlas, to prevent problems like #656, where entities can be placed on the very fringe of the map circle, but because of pathfinder passability they get stuck. Of course it should still be possible to place objects anywhere on the map for unusual circumstances, but as an option, we can add placement constraints. For instance boats shouldn't be built on land, most buildings shouldn't be built in water. I think this can all be done by adding some logic to the create object handler to make it check pathfinding classes, similar to what building restrictions and unit spawning already do in-game. Though that wouldn't work for gaia objects like trees unless we assign them some default passability, maybe there's a better way to handle it.

As a visual indicator, we can apply some shading to the entity preview if it's hovering an invalid location, like we do for building previews.