title: StringFormat and Multibinding with Label
body: <p>I would like to use StringFormat to do someting like this :</p>
  
  <pre><code>&lt;Label x:Name="myLabel"&gt;
      &lt;Label.Content&gt;
          &lt;Multibinding StringFormat="{}{0} - {1}"&gt;
              &lt;Binding Path="Lib1" /&gt;
              &lt;Binding Path="Lib2" /&gt;
          &lt;/MultiBinding&gt;
      &lt;/Label.Content&gt;
  &lt;/Label&gt;
  </code></pre>
  
  <p>However, it's doesn't work and I got this error instead : </p>
  
  <blockquote>
    <p>MultiBinding failed because it has no valid Converter. MultiBindingExpression:target element is 'Label' (Name='myLabel'); target property is 'Content' (type 'Object')</p>
  </blockquote>
  
  <p>Is there any way to make this code work ?</p>