Add chapter 4 completed solution

Chapter04/Complete/MyMediaCollection/Views/MainPage.xaml.cs

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+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.UI.Xaml.Controls;
+using MyMediaCollection.ViewModels;
+
+namespace MyMediaCollection.Views
+{
+    /// <summary>
+    /// An empty page that can be used on its own or navigated to within a Frame.
+    /// </summary>
+    public sealed partial class MainPage : Page
+    {
+        public MainPage()
+        {
+            ViewModel = App.HostContainer.Services.GetService<MainViewModel>();
+            this.InitializeComponent();
+        }
+
+        public MainViewModel ViewModel;
+    }
+}