title: GWT DataGrid Selection Handler
body: <p>I have a DataGrid, and I want to get the selected row when it is clicked:
  I want to get its content and its index.
  I have some sort of a form under the DataGrid, and when one row is selected, the form will be filled with the data in the selected row above!
  Then when I click a button a DialogBox should be shown with the data in the selected row!</p>
  
  <p>I have searched but there is no clear explanation about how to do it.
  Thank you</p>