Handle nested item comparisons, add Equals tests

SabreTools.DatFiles/DatHeader.cs

@@ -370,6 +370,40 @@ namespace SabreTools.DatFiles
 
         #endregion
 
+        #region Comparision Methods
+
+        /// <inheritdoc/>
+        public override bool Equals(ModelBackedItem? other)
+        {
+            // If other is null
+            if (other == null)
+                return false;
+
+            // If the type is mismatched
+            if (other is not DatHeader otherItem)
+                return false;
+
+            // Compare internal models
+            return _internal.EqualTo(otherItem._internal);
+        }
+
+        /// <inheritdoc/>
+        public override bool Equals(ModelBackedItem<Models.Metadata.Header>? other)
+        {
+            // If other is null
+            if (other == null)
+                return false;
+
+            // If the type is mismatched
+            if (other is not DatHeader otherItem)
+                return false;
+
+            // Compare internal models
+            return _internal.EqualTo(otherItem._internal);
+        }
+
+        #endregion
+
         #region Manipulation
 
         /// <summary>