title: add menu item to default context menu
body: <p>I'd like to add a menu item to the default <code>ContextMenu</code> of a <code>RichTextBox</code>.</p>
  
  <p>I could create a new context menu but then I lose the spell check suggestions that show up in the default menu.</p>
  
  <p>Is there a way to add an item without re-implementing everything?</p>