title: ag-Grid onCellEditingStopped event does not provide previous value
body: <p>I am using ag-Grid <strong>onCellEditingStopped</strong> event handler to get the changed value of a grid cell. </p>
  
  <pre><code>onCellEditingStopped: function(event) {
      // event.value present the current cell value
      console.log('cellEditingStopped');
  }
  </code></pre>
  
  <p>But it does not provide the previous value (the value before the change happens). Is there anyway to get the previous value ?</p>
  
  <p>My current solution:</p>
  
  <p>I am using <strong>onCellEditingStarted</strong> event to store the current cell value in a separate variable and use that variable inside the <strong>onCellEditingStopped</strong> event handler function. But it is not a clear solution.</p>
  
  <p>Thanks</p>