title: Adding Gtk::Entry to Gtk::Dialog
body: <p>I am adding a <code>Gtk::Entry</code> to <code>Gtk::Dialog</code>.</p>
  
  <pre><code>Gtk::Dialog *dialog = manage ( new Gtk::Dialog() );
  dialog-&gt;set_title("Add Text");
  
  Gtk::Entry entry;
  
  entry.set_activates_default(true);
  entry.set_max_length(50);
  entry.set_text("hello world");
  entry.select_region(0, entry.get_text_length());
  
  dialog-&gt;add(entry);
  dialog-&gt;show();
  </code></pre>
  
  <p>This code shows an empty dialog box. On using <code>show_all()</code> or <code>show_all_children()</code>, nothing happens, not even an empty dialog box shows up.</p>
  
  <p>And I get the following warning in all cases:</p>
  
  <p><code>Gtk-WARNING **: Attempting to add a widget with type gtkmm__GtkEntry to a gtkmm__GtkDialog, but as a GtkBin subclass a gtkmm__GtkDialog can only contain one widget at a time; it already contains a widget of type GtkVBox</code></p>
  
  <p>Help me resolve this issue.</p>