support for .NET framework 3.5

Still have bugs

Net35/HandyControl_Net35/Controls/Attach/BorderElement.cs

@@ -0,0 +1,14 @@
+using System.Windows;
+
+namespace HandyControl.Controls
+{
+    public class BorderElement
+    {
+        public static readonly DependencyProperty CornerRadiusProperty = DependencyProperty.RegisterAttached(
+            "CornerRadius", typeof(CornerRadius), typeof(BorderElement), new FrameworkPropertyMetadata(default(CornerRadius), FrameworkPropertyMetadataOptions.Inherits));
+
+        public static void SetCornerRadius(DependencyObject element, CornerRadius value) => element.SetValue(CornerRadiusProperty, value);
+
+        public static CornerRadius GetCornerRadius(DependencyObject element) => (CornerRadius)element.GetValue(CornerRadiusProperty);
+    }
+}