Add completed solution for chapter 3

Chapter03/Complete/MyMediaCollection/MainWindow.xaml.cs

@@ -0,0 +1,18 @@
+using Microsoft.UI.Xaml;
+using MyMediaCollection.ViewModels;
+
+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();
+        }
+
+        public MainViewModel ViewModel => App.ViewModel;
+    }
+}