title: Filling the parent with Qt widgets
body: <p>I'd like to create an application's main window in qt4 designer.</p>
  
  <p>When I add a layout element to the <code>centralwidget</code>, that element stays in the middle of the window and does not move when the window is resized. How can I make it "fill" the whole area (and resize its children properly)? All children widgets of my layout are expanding properly. </p>