support for .NET framework 3.5

Still have bugs

Net35/HandyControl_Net35/Tools/Converter/Object2BooleanConverter.cs

@@ -0,0 +1,16 @@
+using System;
+using System.Globalization;
+using System.Windows.Data;
+
+namespace HandyControl.Tools.Converter
+{
+    public class Object2BooleanConverter : IValueConverter
+    {
+        public object Convert(object value, Type targetType, object parameter, CultureInfo culture) => !(value is null);
+
+        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            throw new NotImplementedException();
+        }
+    }
+}