Tank horde

Cities that you own now build tanks for you. Capturing a city uses one tank, then a few turns later, a new tank appears. As you can see from the screenshot, this quickly turns into a whole lot of tanks.

I also added a "sleep key". Press "z", and the selected tank will go to sleep. Press "z" again to wake up the tank. Be careful though: at the end of the turn, any sleeping tanks are skipped, even if they didn't move yet!

The extra tanks and the sleep mean it's time to think about my UI again. For example: how do I let you know a given tank is sleeping, or has no more moves? How do I let you cycle through your active units instead of hunting and clicking?