title: Converter only firing once and not every time label content changes
body: <p>I have label with a width of auto, that is bound to a property of type string. </p>
  
  <pre><code>&lt;Label x:Name="ExampleLabel" Content="{Binding ExampleProperty}"Height="30" Width="Auto" &gt;
  </code></pre>
  
  <p>I then have a property that is bound to the width of that label. The converter should convert the width to a negative value.</p>
  
  <pre><code>&lt;UserControl.Resources&gt;
          &lt;c:PositiveToNegativeConverter x:Key="PositiveToNegativeConverter"/&gt;
  &lt;/UserControl.Resources&gt;
  
  "{Binding ElementName=ExampleLabel, Path=Width, Converter={StaticResource PositiveToNegativeConverter}}"
  </code></pre>
  
  <p>I want the converter to execute when the label content changes, but it is only fired once, when the application loads.</p>
  
  <p>Here is my converter</p>
  
  <pre><code>public class PositiveToNegativeConverter : IValueConverter
  {
      public object Convert(object value, Type targetType,
          object parameter, CultureInfo culture)
      {
          return (double)value * -1;
      }
  
      public object ConvertBack(object value, Type targetType,
          object parameter, CultureInfo culture)
      {
          return Math.Abs((double) value);
      }
  }
  </code></pre>
  
  <p>Any help is much appreciated. Thanks.</p>