Fix a critical warning in notebook dnd with rgba colormaps

Pointed out by Nick Schermer in
https://bugzilla.gnome.org/show_bug.cgi?id=690788

gtk/gtknotebook.c

@@ -3420,6 +3420,7 @@ gtk_notebook_drag_begin (GtkWidget        *widget,
   priv->dnd_window = gtk_window_new (GTK_WINDOW_POPUP);
   gtk_window_set_screen (GTK_WINDOW (priv->dnd_window),
                          gtk_widget_get_screen (widget));
+  gtk_widget_set_colormap (priv->dnd_window, gtk_widget_get_colormap (widget));
   gtk_container_add (GTK_CONTAINER (priv->dnd_window), tab_label);
   gtk_widget_set_size_request (priv->dnd_window,
 			       priv->detached_tab->allocation.width,