Make province borders smarter

[mmyers]

The current border-detection algorithm in editor.MapData is rather crude. It would be nice to extend or re-implement it to provide:

1. Knowledge of which provinces the border divides. This would allow drawing borders around a group of provinces, such as a country or a region, without drawing the internal borders. As a result, it would be much less of an issue if two adjacent areas are colored the same color.
2. Vectorizing the borders. Currently the border is nothing but a set of pixels on the map image, which makes it behave poorly when the map is zoomed in or out. If the borders were lines from one place to another, they would scale much better.