title: QWidget - set border from C++ code
body: <p>I have QWidget instance (with other QWidgets inside) in Qt 5.8 and I want to set some border around it. Is there some way to do this from C++ without affecting any of it's children and their settings and positions?</p>
  
  <p>I don't want to use stylesheets and it needs to work with any system style.</p>