Reorganization of files

App/ContentDialogExample.xaml.cs

@@ -0,0 +1,21 @@
+using System.Windows;
+
+namespace ModernWpfPlayground
+{
+    public partial class ContentDialogExample
+    {
+        public static readonly DependencyProperty MessageProperty = DependencyProperty.Register(
+            "Message", typeof(string), typeof(ContentDialogExample), new PropertyMetadata(default(string)));
+
+        public string Message
+        {
+            get => (string) GetValue(MessageProperty);
+            set => SetValue(MessageProperty, value);
+        }
+
+        public ContentDialogExample()
+        {
+            InitializeComponent();
+        }
+    }
+}