Tank hero

I've updated the game to allow you to capture the other cities with your tank. This "uses up" the tank. In old Strategic Conquest games, cities built units. Since I don't have that yet, I instead magically spawn another tank.

It looks like this:

One challenge I encountered: removing the tank from the board means that there are no human-controlled units left, so the turn automatically ends. This means the end-of-turn popup window never closes, and you see turn counters rapidly incrementing. It looks really weird!

You don't see the weird popup behavior now, because I automatically spawned a tank. However in the future if neither AI nor human have any active units, it could happen again. Perhaps I need to check for this and show "no active units for AI or human; ending turn now".