support for .NET framework 3.5

Still have bugs

Net35/HandyControl_Net35/Shell/ThumbButtonInfoCollection.cs

@@ -0,0 +1,29 @@
+namespace Microsoft.Windows.Shell
+{
+    using System.Windows;
+
+    public class ThumbButtonInfoCollection : FreezableCollection<ThumbButtonInfo>
+    {
+        private static ThumbButtonInfoCollection s_empty;
+
+        protected override Freezable CreateInstanceCore()
+        {
+            return new ThumbButtonInfoCollection();
+        }
+
+        internal static ThumbButtonInfoCollection Empty
+        {
+            get
+            {
+                if (s_empty == null)
+                {
+                    ThumbButtonInfoCollection infos = new ThumbButtonInfoCollection();
+                    infos.Freeze();
+                    s_empty = infos;
+                }
+                return s_empty;
+            }
+        }
+    }
+}
+