Setting up source code for Avalonia port.

src/HistoryItem.cs

@@ -1,19 +0,0 @@
-using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
-
-namespace AdvancedCalculator
-{
-    public class HistoryItem
-    {
-        #region Properties
-
-        public string Input { get; set; }
-
-        public string Output { get; set; }
-
-        #endregion
-    }
-}