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

src/main/java/gg/malloc/defense/ui/Items.java

@@ -0,0 +1,15 @@
+package gg.malloc.defense.engine;
+
+import org.bukkit.inventory.ItemStack;
+import org.bukkit.inventory.meta.ItemMeta;
+import org.bukkit.Material;
+
+public class Items {
+  static ItemStack makeBombHelmet() {
+      ItemStack bombItem = new ItemStack(Material.CARVED_PUMPKIN);
+      ItemMeta meta = bombItem.getItemMeta();
+      meta.setCustomModelData(0);
+      bombItem.setItemMeta(meta);
+      return bombItem;
+  }
+}