Prevent attack another player's structures

[nvs]

First off, if a structure cannot move out of its owner's area as in #197, then there should be zero need to be able to attack another player at all.

Currently, if you attack another player with a structure, then the structure is removed from the game.

For the most part, prevention of slates going into other areas addresses some of this issue. Curiously, Elder slate seems able to destroy rocks due to its AoE. Other sources of AoE may also be issues of concern.

[nvs]

The existing behavior of removing the offending player's structure needs to be adjusted. It should simply prevent the action, and perhaps issue a warning. However, adjusting structure removal in #214, and not forcing the player to attack to achieve such behavior, would allow all of them to be invulnerable. This would solve the issue as well.