Reimplement wave AI, split out some UI code into a ui module, update TODO

src/main/java/gg/malloc/defense/engine/PlayerManager.java

@@ -27,6 +27,10 @@ public class PlayerManager {
     return allSuccess;
   }
 
+  public boolean contains(Player player) {
+    return m_playerStates.containsKey(player);
+  }
+
   public boolean requestTransition(Player player, State toState) {
     State currentState = m_playerStates.get(player);
     if (currentState == toState) {