title: How to change the template from Widget to Main Window?
body: <p>When I started my application, I chose a wrong template in Qt Designer.</p>
  
  <p>I did File -> New -> Widget.</p>
  
  <p>Now I need a menu. So, I need the Main Window template, I suppose.</p>
  
  <p>Can I change the template seamless or I have to create new form and drug everything onto it?</p>