title: How to add QLine to QGridLayout
body: <p>I'm trying to develop a Tic Tac Toe game, in which i'll place a QGridLayout in the QMainWindow central widget. There, I intend on adding other widgets (board cells) and the lines separating the game cells.</p>
  
  <p>Is this possible? I can't find any API to insert a QLine inside a QGridLayout..</p>
  
  <p>If this is not possible, can I place the child widgets directly on the QMainWindow's central widget? If so, how?</p>