title: How to close first windows , and open different window but in the same application (like Skype)
body: <p>Hello all
  Until now I was developing Qt MainWindow and always stay in this window , changing the content with QStackedWidget<br>
  But now I need to do something like to skype ,   the flow is 
  user   open first window the login window after successful login , the window closes<br>
  And new different window opens with the user friends , now maybe it’s the same first window
  That changed its look totally .. I don’t know.<br>
  How I manage this in Qt ? </p>