title: add a scrollbar to a window in LablGtk2
body: <p>I want to add a scrollbar to a window in LablGtk2.
  I wrote as below.</p>
  
  <pre><code> let w = GWindow.window
           ~width:width
           ~height:height
           ~allow_grow:true
           ~allow_shrink:true
           ~decorated:true
           () in
   ignore(GRange.scrollbar `HORIZONTAL ~packing:w#add ~show:true ());
  </code></pre>
  
  <p>However, a window(w in the code) did't show a scrollbar.
  How can I make a scrollbar?</p>