title: gtk: set active window
body: <p>I have a <code>gtk.Window</code>. How do I set it to be the active window? I can call <code>is_active()</code> to see whether it already is, but I don't see where to make it active.</p>
  
  <p>Bonus points: given a <code>gtk.Widget</code>, how do I make the window it is a part of the active window?</p>