fix(gui prefab): 修复两个预制体的精灵帧和配置错误

1. 将ibox.prefab的_spriteFrame设为null，移除无用的uuid引用
2. 修正melist.prefab的颜色值、精灵帧资源和尺寸模式
3. 为melist补全关联的精灵图集引用

assets/resources/gui/element/melist.prefab

@@ -354,18 +354,18 @@
     "_dstBlendFactor": 4,
     "_color": {
       "__type__": "cc.Color",
-      "r": 0,
-      "g": 0,
-      "b": 0,
-      "a": 88
+      "r": 255,
+      "g": 255,
+      "b": 255,
+      "a": 255
     },
     "_spriteFrame": {
-      "__uuid__": "cb93c900-b440-4571-91d1-7da1636e3d73@1be8d",
+      "__uuid__": "cb93c900-b440-4571-91d1-7da1636e3d73@b9538",
       "__expectedType__": "cc.SpriteFrame"
     },
     "_type": 0,
     "_fillType": 0,
-    "_sizeMode": 1,
+    "_sizeMode": 0,
     "_fillCenter": {
       "__type__": "cc.Vec2",
       "x": 0,
@@ -375,7 +375,10 @@
     "_fillRange": 0,
     "_isTrimmedMode": true,
     "_useGrayscale": false,
-    "_atlas": null,
+    "_atlas": {
+      "__uuid__": "cb93c900-b440-4571-91d1-7da1636e3d73",
+      "__expectedType__": "cc.SpriteAtlas"
+    },
     "_id": ""
   },
   {