title: How to allow window resize after calling QMainWindow.setFixedSize?
body: <p>I would like to know how to allow the user to resize the window after calling <code>QMainWindow.setFixedSize</code> ?
  For example, he click a button, that execute a function that re-enable the window resizement.
  I checked on the internet, but I didn't found anything... I tried <code>QMainWindow.setSizePolicy(QSizePolicy.Maximum,QSizePolicy.Minimum)</code>, but that didn't work... Any idea ?</p>
  <p>Thank you !</p>