[DO NOT MERGE] 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.

Windows and doors will display messages indicating extant of damage as they are damaged (e.g. "Cracks begin to appear in [src]!")

Metal rods can substitute for crowbars to force unpowered and broken doors, as well as firelocks.

Window damage code cleaned up and centralized.
I dunno, for me, 4 shots seems pretty low. Considering that you are bypassing most other ways of opening a door it would seem much more fair if you had to use most of your clip, if not all. As well, that four shots is just for laser carbines/energy guns, let alone shotguns or SMGs, which have carry a higher total damage amount in a clip.

How does this sound? If you lower the total health to 300 and take away the multiplier, then you get:

Weapon | Number of Shot to Break
------------|----------------------------------
C20r SMG | 15
Laser Carbine | 8
Shotgun | 5
Laser Cannon | 4

Keep in mind also that the player doesn't have to rely on a single tool to do the job. He can fire a few shots to weaken the door and dramatically reduce the time it takes to break through with a fire extinguisher. 

Reducing the total health from 500 to 300 also shouldn't make too much of a difference for melee weapons in my opinion, as most improvised battering rams will deal 10-15 damage anyway.