title: How can I round a QWidgets corners?
body: <p>I am trying to round the corners of the current widget, but it doesnt work, why?</p>
  
  <pre><code>
  PopupWindow::PopupWindow() : QWidget(0) {
      setWindowFlags(
          Qt::Tool |
          Qt::FramelessWindowHint |
          Qt::WindowSystemMenuHint |
          Qt::WindowStaysOnTopHint
      );
      this->resize(300, 100);
      setStyleSheet(".PopupWindow {border-style: outset;border-width: 10px;border-radius:10px;}");
  
      QPushButton *hello = new QPushButton("Hello world!");
      hello->setFont(QFont("Times", 18, QFont::Bold));
      hello->setGeometry(10, 40, 180, 40);
      hello->setStyleSheet("");
  
  
      QVBoxLayout *layout = new QVBoxLayout;
      layout->addWidget(hello);
      setLayout(layout);
  }
  </code></pre>