support for .NET framework 3.5

Still have bugs

Net35/HandyControlDemo_Net35/Window/AboutWindow.xaml.cs

@@ -0,0 +1,38 @@
+using System.Diagnostics;
+using System.Reflection;
+using System.Windows;
+
+namespace HandyControlDemo.Window
+{
+    public partial class AboutWindow
+    {
+        public AboutWindow()
+        {
+            InitializeComponent();
+
+            DataContext = this;
+
+            var versionInfo = FileVersionInfo.GetVersionInfo(Assembly.GetEntryAssembly().Location);
+            CopyRight = versionInfo.LegalCopyright;
+            Version = $"v {versionInfo.FileVersion}";
+        }
+
+        public static readonly DependencyProperty CopyRightProperty = DependencyProperty.Register(
+            "CopyRight", typeof(string), typeof(AboutWindow), new PropertyMetadata(default(string)));
+
+        public string CopyRight
+        {
+            get => (string) GetValue(CopyRightProperty);
+            set => SetValue(CopyRightProperty, value);
+        }
+
+        public static readonly DependencyProperty VersionProperty = DependencyProperty.Register(
+            "Version", typeof(string), typeof(AboutWindow), new PropertyMetadata(default(string)));
+
+        public string Version
+        {
+            get => (string) GetValue(VersionProperty);
+            set => SetValue(VersionProperty, value);
+        }
+    }
+}