title: GTk3 python Gtk.CellRendererText() -> Warning Refusing to add the same cell renderer
body: <p>As heading says: a small seemingly ok snippet generates 3 warnings:</p>
  
  <pre><code>Gtk-WARNING **: Refusing to add the same cell renderer to a GtkCellAreaBox twice
  </code></pre>
  
  <p>The code:</p>
  
  <pre><code>from gi.repository import Gtk
  renderers = {}
  for ix, col in enumerate(['vendor', 'conf', 'supports']):
      renderers[col] = Gtk.CellRendererText()
      column = Gtk.TreeViewColumn(col, renderers[col], text=ix)
      column.pack_start(renderers[col], True)
  </code></pre>
  
  <p>It seems to work anyway in it's natural context, but I'm not really happy with these warnings... tested with both python2 and python3, no difference (as expected) </p>
  
  <p><strong>Gtk version:</strong> gtk3-3.10.6-1.fc20.i686</p>
  
  <p>Any hint out there?</p>
  
  <p>--alec</p>