Turn Churn

I have added a turn-taking system into the game. It doesn't add much noticeable effect at the moment, because there's only one unit, and the turn automatically advances once the unit is done moving. You'll see a brief pause and maybe hear the "denied" sound every once in a while.

I also added a HUD-style UI whenever a turn ends, showing you the turn number that just completed, plus the next turn number. In the old strategic conquest game, this number affected the strength of bombers. I found bombers annoying, but I will nonetheless be adding bombers, and then the turn numbers will have an in-game effect.

The turn-based system was conceptually difficult for me. I had to mentally connect the idea of using the `_process` or `_physics_process` loop to the turn system. Time deltas can  ping the turn state at any time. So I have to set flags to track which part of turn checking I am on. I may have to revisit that later, because flags seems fragile. Something something "state machine".