title: How to put a shadow to a Frameless window in Qt
body: <p>I'm trying to create a Frameless window that has a shadow under it.When we create a borderless window with <strong>Qt::FramelessWindowHint</strong> flag it doesn't contain a shadow.
  But we can put shadows to a child widgets easy by creating a <strong>QGraphicsDropShadowEffect</strong> object and then pass it to the widget through <strong>setGraphicsEffect</strong> function. But this doesn't seem to work for QMainWindow. Please help me to put shadow to a frameless window in Qt...  </p>