title: DevExpress: Group items by some property in ComboBox
body: <p>I have a <code>DevExpress</code> element in my project - <code>GridView</code>. And there's a ComboBox column. 
  It has a DataSource property assigned to a collection of some items. So I need to group these items by a specific field. </p>
  
  <p>It should look like this:</p>
  
  <pre><code>Group 1
    - item 1
    - item 2
    - item 3
  Group 2
    - item 4
    - item 5
  </code></pre>
  
  <p>Could you help me how can I implement this? Thank you!</p>