Adds false bottom briefcases and an IAA traitor item.

As inspired by an anon in the general:
False bottom briefcases! Use a screwdriver to pop open a secret panel, which allows you to store an item that's invisible to a quick search. If you store a gun in the bottom, you can fire it by waving the briefcase angrily at another person.

The IAA gets a traitor item which is meant to be a useful assassination tool in the right hands: a false bottom briefcase with a starting SMG. Stealthy, but a pain to reload or put on burstfire since it's normally trapped in the briefcase. 6 TC, since it's a good weapon.
Storage items work by looking at their contents and showing it all to the player. If I were to make this `item.loc = src`, the hidden item would show up in the menu whenever the player opened the briefcase, which rather defeats the point.