title: Add button to comboBox with MVVM
body: <p>I am using an MVVM pattern and have a <code>ComboBox</code> that binds to properties in the viewmodel like this:</p>
  
  <pre><code>&lt;ComboBox ItemsSource="{Binding Path=ItemCollection}"
            SelectedItem="{Binding Path=SelectedItem}"&gt;    
      &lt;ComboBox.ItemTemplate&gt;
          &lt;DataTemplate&gt;
               &lt;!-- Custom combobox item template --&gt;
          &lt;/DataTemplate&gt;
      &lt;/ComboBox.ItemTemplate&gt;
  &lt;/ComboBox&gt;
  </code></pre>
  
  <p>This works fine and in the <code>DataTemplate</code> I can specify how each item should be displayed.</p>
  
  <p>I would like to add a button at the end of the combobox dropdown itempresenter.
  Similar to how its done in the image below from MS Word.</p>
  
  <p><a href="https://i.stack.imgur.com/Kl7nK.png" rel="nofollow"><img src="https://i.stack.imgur.com/Kl7nK.png" alt="ComboBox with button"></a> </p>
  
  <p>When the selected button "More Columns..." is pressed a dialog is shown and the user can input details. I am trying to archive the same workflow.</p>