Add chapter 4 completed solution

Chapter04/Complete/MyMediaCollection/MainWindow.xaml.cs

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+using Microsoft.UI.Xaml;
+
+namespace MyMediaCollection
+{
+    /// <summary>
+    /// An empty window that can be used on its own or navigated to within a Frame.
+    /// </summary>
+    public sealed partial class MainWindow : Window
+    {
+        public MainWindow()
+        {
+            this.InitializeComponent();
+        }
+    }
+}