Fix CS1591 warnings

Code CS1591: Missing XML comment for publicly visible type or member.

UnitTests/UnitTests.Notifications.UWP/UnitTestApp.xaml.cs

@@ -16,6 +16,9 @@ namespace UnitTests.Notifications.UWP
     /// </summary>
     public sealed partial class App : Application
     {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="App"/> class.
+        /// </summary>
         public App()
         {
             InitializeComponent();