Rename DefensiveUnit to Structure

This is more descriptive for the way I want to use it.
Make sure all created units have at least 1 hp

I almost "fixed" this in a very incorrect way but removing "is_alive"
check when drawing buildings.
Add Wall class
Ugly rough drawing technique for walls
Properly draw Wall based on being a Structure

Previously I was looking for an exact class match on Wall, because I
thought it was necessary for a reason I didn't understand. Fortunately
it was a problem elsewhere, and I can use the subclassing as I thought.
Add line of walls to defendingList
Set wall positions using array assignment