Extract all converters into separate folder

Wox/Converters/StringNullOrEmptyToVisibilityConverter.cs

@@ -0,0 +1,24 @@
+using System;
+using System.Globalization;
+using System.Windows;
+
+namespace Wox.Converters
+{
+    public class StringNullOrEmptyToVisibilityConverter : ConvertorBase<StringNullOrEmptyToVisibilityConverter>
+    {
+        public override object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            return string.IsNullOrEmpty(value as string) ? Visibility.Collapsed : Visibility.Visible;
+        }
+
+        public override object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            return null;
+        }
+
+        public override object ProvideValue(IServiceProvider serviceProvider)
+        {
+            return this;
+        }
+    }
+}