Overmap areas and city generation implementation

[snipercup]

Requires #412

Having maps use a neighbor_key didn't work out because there is too little control over what spawns where. Instead, I added a new data type called overmaparea. This allows you to select some maps that should spawn in this area some distance from the center. This allows us to create an urban area in the center, the suburban area in the middle and fields in the edges. I started off with a perfect circle for every city, but I added noise to break the shape somewhat.

The spawning on the overmap is still basic and the tacticalmaps get in the way. I removed the neighbor keys from the maps since they are replaced by the overmaparea structure.