Add OfflineList deserialization test, fix issues

SabreTools.Models/OfflineList/GUI.cs

@@ -0,0 +1,24 @@
+using System.Xml;
+using System.Xml.Serialization;
+
+namespace SabreTools.Models.OfflineList
+{
+    [XmlRoot("gui")]
+    public class GUI
+    {
+        [XmlElement("images")]
+        public Images? Images { get; set; }
+
+        #region DO NOT USE IN PRODUCTION
+
+        /// <remarks>Should be empty</remarks>
+        [XmlAnyAttribute]
+        public XmlAttribute[]? ADDITIONAL_ATTRIBUTES { get; set; }
+
+        /// <remarks>Should be empty</remarks>
+        [XmlAnyElement]
+        public object[]? ADDITIONAL_ELEMENTS { get; set; }
+
+        #endregion
+    }
+}