support for .NET framework 3.5

Still have bugs

Net35/HandyControl_Net35/Controls/Base/HeaderedSelectableItem.cs

@@ -0,0 +1,16 @@
+using System.Windows;
+
+namespace HandyControl.Controls
+{
+    public class HeaderedSelectableItem : SelectableItem
+    {
+        public static readonly DependencyProperty HeaderProperty = DependencyProperty.Register(
+            "Header", typeof(object), typeof(HeaderedSelectableItem), new PropertyMetadata(default(object)));
+
+        public object Header
+        {
+            get => GetValue(HeaderProperty);
+            set => SetValue(HeaderProperty, value);
+        }
+    }
+}