Controls: Rewrite UnitTextBox as extension of TextBox instead of UserControl

Elwig/Controls/WidthToMarginConverter.cs

@@ -0,0 +1,16 @@
+using System;
+using System.Windows;
+using System.Windows.Data;
+using System.Globalization;
+
+namespace Elwig.Controls {
+    public class WidthToMarginConverter : IValueConverter {
+        public object Convert(object value, Type targetType, object parameter, CultureInfo culture) {
+            return new Thickness(0, 0, 2 + (double)value, 0);
+        }
+
+        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture) {
+            return ((Thickness)value).Right - 2;
+        }
+    }
+}