Christmas theme

I'm starting to implement "sides". There's a red side which is the enemy, and a green side which is you. Each starts with one soldier, plus a neutral city in the middle.

This is really abusing the "unit" type. I need to make a general "entities" type instead, and a dedicated city entity. Right now it is re-using unit code, which means it has nonsensical things like "walk" code. Changing to city type should allow me to also walk a soldier into a city.