Added WPF tester

EonaCat.VolumeMixer.Tester.WPF/Converter/VolumeToPercentageConverter.cs

@@ -0,0 +1,22 @@
+using System.Globalization;
+using System.Windows.Data;
+
+namespace EonaCat.VolumeMixer.Tester.WPF.Converter
+{
+    public class VolumeToPercentageConverter : IValueConverter
+    {
+        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            if (value is float volume)
+            {
+                return $"{Math.Round(volume * 100)}%";
+            }
+            return "0%";
+        }
+
+        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            throw new NotImplementedException();
+        }
+    }
+}