title: Qt add buttons to existing widget
body: <p>I have added two pushButtons using qtDesigner, those buttons are chilldren of centralWidget, now I would like to add one more pushButton, but programmatically in mainwindow.cpp. </p>
  
  <p>When I use setCentralWidget method it removes or hides prevous 2 pushButtons which were added by qtDesigner. My question is how to add this additional button programmatically so those 2 pushButtons remains?</p>
  
  <p>I am beginner in Qt.</p>