Add project files.

WinGreedWPF/ExtraChild.xaml.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using System.Windows;
+using System.Windows.Controls;
+using System.Windows.Data;
+using System.Windows.Documents;
+using System.Windows.Input;
+using System.Windows.Media;
+using System.Windows.Media.Imaging;
+using System.Windows.Shapes;
+
+namespace WinGreedWPF
+{
+    /// <summary>
+    /// Interaction logic for ExtraChild.xaml
+    /// </summary>
+    public partial class ExtraChild : Window
+    {
+        public ExtraChild()
+        {
+            InitializeComponent();
+        }
+    }
+}