Add files for the reactive example

ReactiveExample/Extensions.cs

@@ -0,0 +1,41 @@
+using Terminal.Gui;
+
+namespace ReactiveExample {
+	public static class Extensions
+	{
+		public static MemoizedElement<TOwner, TNew> StackPanel<TOwner, TNew>(
+			this TOwner owner,
+			TNew control)
+			where TOwner : View
+			where TNew : View =>
+			new MemoizedElement<TOwner, TNew>(owner, control);
+
+		public static MemoizedElement<TOwner, TNew> Append<TOwner, TOld, TNew>(
+			this MemoizedElement<TOwner, TOld> owner, 
+			TNew control,
+			int height = 1)
+			where TOwner : View 
+			where TOld : View
+			where TNew : View
+		{
+			control.X = Pos.Left(owner.Control);
+			control.Y = Pos.Top(owner.Control) + height;
+			return new MemoizedElement<TOwner, TNew>(owner.View, control);
+		}
+
+		public class MemoizedElement<TOwner, TControl> 
+			where TOwner : View 
+			where TControl : View
+		{
+			public TOwner View { get; }
+			public TControl Control { get; }
+
+			public MemoizedElement(TOwner owner, TControl control)
+			{
+				View = owner;
+				Control = control;
+				View.Add(control);
+			}
+		}
+	}
+}