support for .NET framework 3.5

Still have bugs

Net35/HandyControl_Net35/Tools/Converter/String2VisibilityConverter.cs

@@ -0,0 +1,20 @@
+using System;
+using System.Globalization;
+using System.Windows;
+using System.Windows.Data;
+
+namespace HandyControl.Tools.Converter
+{
+    public class String2VisibilityConverter : IValueConverter
+    {
+        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            return string.IsNullOrEmpty((string)value) ? Visibility.Collapsed : Visibility.Visible;
+        }
+
+        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            return value != null && (Visibility)value == Visibility.Collapsed;
+        }
+    }
+}