Add Mail editor (WIP)

Update Drop editor indexes

Source/GrasscutterTools/Game/Mail/MailItem.cs

@@ -0,0 +1,28 @@
+namespace GrasscutterTools.Game.Mail
+{
+    /// <summary>
+    /// 附件
+    /// </summary>
+    public struct MailItem
+    {
+        /// <summary>
+        /// 物品ID
+        /// </summary>
+        public int ItemId { get; set; }
+
+        /// <summary>
+        /// 物品数量
+        /// </summary>
+        public int ItemCount { get; set; }
+
+        /// <summary>
+        /// 物品等级
+        /// </summary>
+        public int ItemLevel { get; set; }
+
+        public override string ToString()
+        {
+            return $"{ItemId}:{GameData.Items[ItemId]} x{ItemCount} lv{ItemLevel}";
+        }
+    }
+}