Add starter project for chapter 4

Chapter04/Start/MyMediaCollection/App.xaml.cs

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+using Microsoft.UI.Xaml;
+using MyMediaCollection.ViewModels;
+
+namespace MyMediaCollection
+{
+    /// <summary>
+    /// Provides application-specific behavior to supplement the default Application class.
+    /// </summary>
+    public partial class App : Application
+    {
+        /// <summary>
+        /// Initializes the singleton application object.  This is the first line of authored code
+        /// executed, and as such is the logical equivalent of main() or WinMain().
+        /// </summary>
+        public App()
+        {
+            this.InitializeComponent();
+        }
+
+        public static MainViewModel ViewModel { get; } = new MainViewModel();
+
+        /// <summary>
+        /// Invoked when the application is launched.
+        /// </summary>
+        /// <param name="args">Details about the launch request and process.</param>
+        protected override void OnLaunched(Microsoft.UI.Xaml.LaunchActivatedEventArgs args)
+        {
+            m_window = new MainWindow();
+            m_window.Activate();
+        }
+
+        private Window m_window;
+    }
+}