Added the resource economy, its works and their icons

- Five resources (energy, steel, food, luxury, high-tech) with per-city stores, a world market, delivery and material upkeep.
- Resource-producing tile works built outside cities; every nation opens with the works its economy needs.
- Real game-icons.net art replacing the dummy icons for the new resources and works, with the credits updated.

tests/stacking_test.js

@@ -135,7 +135,7 @@ export class PillageTest extends TestCase {
     this.assertTrue(state.requestPillage([unit.id]));
 
     this.assertApprox(state.tilePopulation.get(k), 90000, 1);
-    this.assertApprox(state.tileGdpPenalty.get(k), 0.7, 1e-9);
+    this.assertApprox(state.tileProductionPenalty.get(k), 0.7, 1e-9);
     this.assertApprox(state.getTileGdpPerCapita(coords), gdpBefore * 0.7, 1e-6);
     this.assertApprox(unit.hp, 110, 1e-9, "heals 1 HP per 1000 people removed");
   }
@@ -153,7 +153,7 @@ export class PillageTest extends TestCase {
     this.assertTrue(state.requestPillage([strong.id, weak.id]));
 
     this.assertApprox(state.tilePopulation.get(k), 81000, 1, "90% of 90% remains");
-    this.assertApprox(state.tileGdpPenalty.get(k), 0.49, 1e-9, "70% of 70% remains");
+    this.assertApprox(state.tileProductionPenalty.get(k), 0.49, 1e-9, "70% of 70% remains");
     this.assertApprox(weak.hp, strong.maxHp / 2 + 10, 1e-9, "the weakest pillages first");
     this.assertApprox(strong.hp, strong.maxHp - 100 + 9, 1e-9, "the second heals for the 9000 it removed");
   }