Add starter and complete projects for ch 7

Chapter07/Complete/MyMediaCollection/MainWindow.xaml.cs

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+using Microsoft.UI;
+using Microsoft.UI.Composition.SystemBackdrops;
+using Microsoft.UI.Windowing;
+using Microsoft.UI.Xaml;
+using System;
+using WinRT.Interop;
+using Microsoft.UI.Xaml.Media;
+
+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
+    {
+        private AppWindow _appWindow;
+        private const string AppTitle = "My Media Collection";
+
+        public MainWindow()
+        {
+            this.InitializeComponent();
+            SystemBackdrop = new MicaBackdrop
+            {
+                Kind = MicaKind.BaseAlt
+            };
+            _appWindow = GetCurrentAppWindow();
+            _appWindow.Title = AppTitle;
+        }
+
+        private AppWindow GetCurrentAppWindow()
+        {
+            IntPtr handle = WindowNative.GetWindowHandle(this);
+            WindowId windowId = Win32Interop.GetWindowIdFromWindow(handle);
+            return AppWindow.GetFromWindowId(windowId);
+        }
+
+        internal void SetPageTitle(string title)
+        {
+            if (_appWindow == null)
+            {
+                _appWindow = GetCurrentAppWindow();
+            }
+
+            _appWindow.Title = $"{AppTitle} - {title}";
+        }
+    }
+}