Cosmetic system overhaul: integrate cosmetic manifest handling and update sprite/icon loading logic

resources/cosmetics/cosmetic_pack/test/manifest.json

@@ -0,0 +1,29 @@
+{
+  "id": "test",
+  "name": "Test",
+  "assets": {
+    "structure": {
+      "img": {
+        "port": "structure/test.png",
+        "city": "structure/test.png",
+        "factory": "structure/test.png",
+        "missilesilo": "structure/test.png",
+        "defensepost": "structure/test.png",
+        "samlauncher": "structure/test.png"
+      }
+    },
+    "sprites": {
+      "transportship": "sprites/test.png",
+      "warship": "sprites/test.png",
+      "sammissile": "sprites/test.png",
+      "atombomb": "sprites/test.png",
+      "hydrogenbomb": "sprites/test.png",
+      "tradeship": "sprites/test.png",
+      "mirv": "sprites/test.png",
+      "engine": "sprites/test.png",
+      "carriage": "sprites/test.png",
+      "loadedcarriage": "sprites/test.png"
+    },
+    "audio": {}
+  }
+}