title: flex datagrid columns drag
body: <p>I have a data grid where users can drag columns and reposition them.
  But there is a strange requirement that some columns should not be draged to the left of some other column.</p>
  
  <p>eg, assume the columns are :  name, price , start date, end date,</p>
  
  <p>The end date should not be dragged and placed before the start date.
  i.e. The user can have  </p>
  
  <ul>
  <li>start date, price , name , end date.</li>
  <li>name, start date, price , end date.</li>
  </ul>
  
  <p>But at no point can end date appear before start date.</p>
  
  <p><strong>Is there a way to do this flex?
  Is there a way to know where the user is trying to drop the column and show error message ?</strong></p>