title: Respond to application-wide "hotkey" in Qt
body: <p>I've got a simple Qt app, and I just want to respond to the <kbd>F12</kbd> key, regardless of which widget has focus.</p>
  
  <p>Is there some easy signal or something I can hook in to?</p>
  
  <p>I want to use the <kbd>F12</kbd> key to toggle the main window fullscreen on/off.</p>