title: QTableWidget, allow to type numbers only
body: <p>I have a <code>QTableWidget</code> with four columns. I want the user to be able to insert only integers in the first three and a double in the fourth.</p>
  
  <p>I believe that this can be done with delegate, but I have not found relevant examples (only some with <code>QDateTimeEdit</code>).</p>
  
  <p>What should I do? </p>