Fix collapsing

1) Stops smashing windows and doors from bringing the ceiling down.
2) Makes squares near walls properly destroy themselves when the ceiling does collapse on them.
3) Made buildings slightly less likely to collapse. (The old system as a whole is still fairly random and inflexible, but it's now much less likely to collapse from taking down a single door, window or wall). That said if you are planning to do some demolition work I'd highly suggest working from the outside, and possibly erecting some walls around the area you plan to take out if it's rather large, especially in buildings that don't have dividing walls (such as evac shelters).

(Maybe a potential new construction would be a "support pillar" that could be quickly erected and removed to help stop things from collapsing while you work?)