(De)constructable emergency shutters.

Can now construct emergency shutter assemblies from metal sheets, same as door assemblies.
Requires air alarm circuits to complete, rather than airlock circuits.

![Right in the middle of the room is the best place for an emergency shutter.](https://www.dropbox.com/s/5iglhneqlr6so7h/EmergencyShutter.png?dl=1)

To construct:
* Metal sheet, at least stack of 4 > Airlock Assemblies > Emergency Shutter Assembly
* Wrench in place where you want it.
* Apply air alarm electronics. 
* Done. Emergency shutter ready for use.

To deconstruct.
* Weld it shut.
* Open the maintenance hatch with a screwdriver.
* Force the circuit out with a crowbar.
* Use a welder to melt down the assembly
* Done. The emergency shutter is gone and you get some of the metal back.