support for .NET framework 3.5

Still have bugs

Net35/HandyControl_Net35/Controls/Attach/IconSwitchElement.cs

@@ -0,0 +1,21 @@
+using System.Windows;
+using System.Windows.Media;
+
+namespace HandyControl.Controls
+{
+    public class IconSwitchElement : IconElement
+    {
+        public static readonly DependencyProperty GeometrySelectedProperty = DependencyProperty.RegisterAttached(
+            "GeometrySelected", typeof(Geometry), typeof(IconSwitchElement), new PropertyMetadata(default(Geometry)));
+
+        public static void SetGeometrySelected(DependencyObject element, Geometry value)
+        {
+            element.SetValue(GeometrySelectedProperty, value);
+        }
+
+        public static Geometry GetGeometrySelected(DependencyObject element)
+        {
+            return (Geometry)element.GetValue(GeometrySelectedProperty);
+        }
+    }
+}