title: Disable native Next button in Qt installer framework
body: <p>I have to disable standard next button, on my custom page via installscript.qs file.</p>
  
  <p>I can disable my own button (that I created in .ui file) via .qs script like this: <code>widget.myButton.setEnabled(false);</code></p>
  
  <p><a href="http://doc.qt.io/qtinstallerframework/scripting-buttons.html" rel="noreferrer">This</a> man shows that native buttons represented as enumeration and I cannot disable them same way. </p>
  
  <p>Controller Scripting manual page shows some interactions with native buttons. Like <code>gui.clickButton(buttons.NextButton)</code>. I go through whole gui object man and don't found anything useful.</p>
  
  <p>Qt installer framework has a native license check page with Next button logic that I need, but I have not found any samples that do it manually. (license page work because its default license page and it's logic inside framework as I understand).</p>
  
  <p>Finally I <a href="http://doc.qt.io/qt-5/qwizardpage.html#isComplete" rel="noreferrer">found</a> isComplete() method that can be useful for me, but it is for C++ API not for qs.
  So how to disable native button via installscript.qs file?</p>