Breakable doors, door and window damage messages

Doors can now be broken through damage. Broken doors can be crowbarred open. High-securiry doors automatically bolt themselves when broken. Doors which are damaged, but not broken, can be repaired with sheet metal. Broken doors must be reconstructed. Spark overlays for damaged and broken doors.

Weapon needs to have force of at least 10 (i.e. fire extinguisher) to damage airlocks. Window doors need only 4 force (i.e. emergency oxygen tank).

Windows and doors will display messages indicating extant of damage as they are damaged (e.g. "Cracks begin to appear in [src]!")

Window damage code cleaned up and centralized.