Add Logiqx deserialization test, fix issues

SabreTools.Models/Logiqx/ItemBase.cs

@@ -0,0 +1,23 @@
+using System.Xml;
+using System.Xml.Serialization;
+
+namespace SabreTools.Models.Logiqx
+{
+    /// <summary>
+    /// Base class to unify the various item types
+    /// </summary>
+    public abstract class ItemBase
+    {
+        #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
+    }
+}