title: Add custom action to System Menu in a QDialog
body: <p>I have a need to add a custom action (say ‘About’ clicking which a QMessageBox needs to be displayed) in the system menu shown when the icon on the title bar of a QDialog is clicked. How do I achieve this?</p>
  
  <p>Regards,
  Bharath</p>
  
  <p><img src="https://i.stack.imgur.com/O0spv.png" alt="Window components"></p>