Controls: Rewrite UnitTextBox as extension of TextBox instead of UserControl

Elwig/App.xaml

@@ -7,7 +7,7 @@
         Exit="Application_Exit">
     <Application.Resources>
         <ctrl:BoolToStringConverter x:Key="BoolToStarConverter" FalseValue="" TrueValue="*"/>
-        <ctrl:WidthToPaddingConverter x:Key="WidthToPaddingConverter"/>
+        <ctrl:WidthToMarginConverter x:Key="WidthToMarginConverter"/>
 
         <DataTemplate x:Key="PostalDestTemplate">
             <StackPanel Orientation="Horizontal">