* Mice no longer spawn on grilles
Fixes#21574. Mice no longer spawn on cables that have dense objects
also on the tile.
Also added the Mice Migration event, used for debug purposes, currently
disabled.
* Uses proc that exists to check for density
* Restructured handle_environment()
This creates the environment_is_safe() proc for simple_animals
* Mice are placed at roundstart randomly
Rejoyce as wires are bitten in random locations.
* Removes all mice from the maps
* Code review
* Code review II
Jack Edge
Cyberboss