title: WPF combobox binding placeholder item
body: <p>I'm trying to add a "fake" item to my source list binded to a combobox.</p>
  
  <pre><code>&lt;Window ...
          DataContext="{Binding Source={x:Static local:Singleton.Instance}}"&gt;
      ...
          &lt;ComboBox ItemsSource="{Binding MyList}"&gt;
  </code></pre>
  
  <p>--</p>
  
  <pre><code>public List&lt;Object&gt; MyList{ get; private set; }
  </code></pre>
  
  <p>I want to have a "Add New" as a combobox item that do not belong to MyList,
  since I need to have only proper objects inside it.
  If I try to add it programmatically an exception is raised since the source cannot be edit in such a way.</p>