Add the ability to modify tiles, monster health, etc, in wizard mode

[nstoddard]

This PR makes it much easier to debug some issues and test some PRs, but it's not quite enough for others. Being able to modify tiles would be helpful for debugging issues like #481, by making it possible to place a pressure plate or a chasm tile. Issues like #502 would be a little easier to debug if there were a way to modify a monster's health (using a staff works too, but the damage is inconsistent and for some issues it might be hard to reach the exact health level required to repro the issue).

These two cases, and others, could be handled with a new wizard mode UI that lets you select a tile and modify anything on that tile. If you select a tile with a monster, you could modify the monster's health and maybe other characteristics. If you select an empty tile, you could replace it with a different tile type, place a trap, etc.