support for .NET framework 3.5

Still have bugs

Net35/HandyControl_Net35/Tools/Helper/DesignerHelper.cs

@@ -0,0 +1,22 @@
+using System.ComponentModel;
+using System.Windows;
+
+namespace HandyControl.Tools
+{
+    public class DesignerHelper
+    {
+        private static bool? _isInDesignMode;
+
+        public static bool IsInDesignMode
+        {
+            get
+            {
+                if (!_isInDesignMode.HasValue)
+                {
+                    _isInDesignMode = (bool)DependencyPropertyDescriptor.FromProperty(DesignerProperties.IsInDesignModeProperty, typeof(FrameworkElement)).Metadata.DefaultValue;
+                }
+                return _isInDesignMode.Value;
+            }
+        }
+    }
+}