title: Get event or notification when user click in taskbar/dock icon in Qt application
body: <p>I'm developing an application in osx, windows, linux</p>
  
  <p>I want to make feature like skype. when user click close windows, application not exit but hidden. When user click appicon on dock or taskbar, my mainwindow will re-open again.</p>
  
  <p>How do I get a notification or filter events to known when user click on app icon?</p>