title: Extjs 4 grid: Add a checkbox column without attaching to a datafield
body: <p>I need to have a column in my Extjs grid with a checkbox which doesn't have a data field assigned to it. I have tried checkColumn plugin but it only works when I assign a data field to it.</p>
  
  <p>What I mean is that it works for the following code:</p>
  
  <pre><code>{header: 'Business Call?',dataIndex: 'business', width: 55, xtype: 'checkcolumn'}
  </code></pre>
  
  <p>But for this code, it only displays a checkbox, but it's not clickable:</p>
  
  <pre><code>{header: 'Set Business Call', width: 55, xtype: 'checkcolumn'}
  </code></pre>
  
  <p>What I want to happen is, when the user ticks the checkbox, I need to do some operation on the respective database row. Is this possible with Extjs?</p>