title: using a radio group in a extjs grid
body: <p>I was using a <code>new Ext.grid.CheckColumn</code> in my EXTJS <code>FlexGridPanel</code><br>
   Now the column of checkboxes do not allow multiple selections , so I need to replace it with a column of radio buttons. 
  Is it possible to make the CheckColumn as single-select?<br>
  If not, how can I put the column of radio buttons. In this link, it seems like the grid does not support radio buttons.<br>
  <a href="http://docs.sencha.com/ext-js/3-4/#!/api/Ext.grid.CheckboxSelectionModel" rel="nofollow">http://docs.sencha.com/ext-js/3-4/#!/api/Ext.grid.CheckboxSelectionModel</a></p>
  
  <p>I say this because there is <code>CheckboxSelectionModel</code> but no <code>RadioButtonSelectionModel</code></p>