add Screenshot Demo

src/Shared/HandyControlDemo_Shared/UserControl/Controls/ScreenshotDemoCtl.xaml

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+<UserControl x:Class="HandyControlDemo.UserControl.ScreenshotDemoCtl"
+             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:hc="https://handyorg.github.io/handycontrol"
+             xmlns:langs="clr-namespace:HandyControlDemo.Properties.Langs"
+             Background="{DynamicResource RegionBrush}">
+    <hc:TransitioningContentControl>
+        <Button Content="{x:Static langs:Lang.StartScreenshot}" Command="hc:ControlCommands.StartScreenshot"/>
+    </hc:TransitioningContentControl>
+</UserControl>

src/Shared/HandyControlDemo_Shared/UserControl/Controls/ScreenshotDemoCtl.xaml.cs

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+using System;
+using System.Windows;
+using System.Windows.Controls;
+using System.Windows.Media;
+using HandyControl.Controls;
+using HandyControl.Data;
+
+namespace HandyControlDemo.UserControl
+{
+    public partial class ScreenshotDemoCtl : IDisposable
+    {
+        public ScreenshotDemoCtl()
+        {
+            InitializeComponent();
+            Screenshot.Snapped += Screenshot_Snapped;
+        }
+
+        private void Screenshot_Snapped(object sender, FunctionEventArgs<ImageSource> e)
+        {
+            new HandyControl.Controls.Window
+            {
+                Content = new Image
+                {
+                    Source = e.Info,
+                    Stretch = Stretch.None
+                },
+                WindowStartupLocation = WindowStartupLocation.CenterScreen
+            }.ShowDialog();
+        }
+
+        public void Dispose() => Screenshot.Snapped -= Screenshot_Snapped;
+    }
+}