Add extension property for backup header

SabreTools.Serialization/Wrappers/N3DS.cs

@@ -16,6 +16,11 @@ namespace SabreTools.Serialization.Wrappers
 
         #region Extension Properties
 
+        /// <summary>
+        /// Backup header
+        /// </summary>
+        public NCCHHeader? BackupHeader => Model.CardInfoHeader?.InitialData?.BackupHeader;
+
         /// <summary>
         /// Backup Write Wait Time (The time to wait to write save to backup after the card is recognized (0-255
         /// seconds)). NATIVE_FIRM loads this flag from the gamecard NCSD header starting with 6.0.0-11.