title: QT: how to open a new window
body: <p>Well, I tried a lot. 
  For example,</p>
  
  <pre><code>#include "secdialog.h"
  
  buttton ::on clicked()
  {
     secdialog  secdialog;
     secdialog.setmodal(true);
     secdialog.exec();
  }
  </code></pre>
  
  <p>Well, secdialog does not exist!
  Those posts are old at least 5 years.
  So, I think some headers are changed or I missed something to add.
  Hopefully, I'll find someone to help me.
  Easy example where to add headers and where to write...</p>