Add Mail editor (WIP)

Update Drop editor indexes

Source/GrasscutterTools/Game/Mail/Mail.cs

@@ -0,0 +1,49 @@
+using System;
+using System.Collections.Generic;
+
+using Newtonsoft.Json;
+
+namespace GrasscutterTools.Game.Mail
+{
+    /// <summary>
+    /// 邮件
+    /// </summary>
+    public class Mail
+    {
+        /// <summary>
+        /// 发件人
+        /// </summary>
+        public string Sender { get; set; }
+
+        /// <summary>
+        /// 收件人 (0 表示发送给所有人)
+        /// </summary>
+        public int Recipient { get; set; }
+
+        /// <summary>
+        /// 是否发送给所有人
+        /// </summary>
+        [JsonIgnore]
+        public bool SendToAll => Recipient == 0;
+
+        /// <summary>
+        /// 邮件标题
+        /// </summary>
+        public string Title { get; set; }
+
+        /// <summary>
+        /// 邮件内容
+        /// </summary>
+        public string Content { get; set; }
+
+        /// <summary>
+        /// 附件列表
+        /// </summary>
+        public List<MailItem> ItemList { get; set; }
+        
+        /// <summary>
+        /// 发送时间
+        /// </summary>
+        public DateTime SendTime { get; set; }
+    }
+}

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}";
+        }
+    }
+}