Extract all converters into separate folder

Wox/Converters/StringEmptyConverter.cs

@@ -0,0 +1,18 @@
+using System;
+using System.Globalization;
+
+namespace Wox.Converters
+{
+    public class StringEmptyConverter : ConvertorBase<StringEmptyConverter>
+    {
+        public override object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            return string.IsNullOrEmpty((string)value) ? parameter : value;
+        }
+
+        public override object ProvideValue(IServiceProvider serviceProvider)
+        {
+            return this;
+        }
+    }
+}