New defense unit - Barricades (+ more)

Today I spent a lot of time refactoring code to make the internals of this thing cleaner and reduce the amount of code duplication. This was supposed to be a purely cosmetic set of changes, but in fact after I cleaned everything up I found that it was much easier to spot inefficiencies and to add new features - this is the magic of clean code, I suppose.

Anyway, update notes:

 - Barricades have been added to the game. They can withstand about 10 hits from zombies before disintegrating, but civilians and bullets can pass through them, so they work great in conjunction with a sniper tower or two.

 - Yet more performance optimisations. As a part of these optimisations, the "vision" hitboxes of all entites have been changed from circles to squares - you may notice that your sniper towers now have a longer range when shooting diagonally as a result. The performance improvement from making Zombie and Civilian viewboxes square is huge, but since there are relatively few defences in comparison I might revert sniper towers back to circular ranges in the future.