title: PyGObject GtkFileChooserDialog default button
body: <p>When I create a <code>GtkFileChooserDialog</code> in Glade and then connect that to a Python class using <code>Gtk.Builder</code>, how do I tell it which is the default button?</p>
  
  <p>Currently, my 'Open' button is the stock <code>gtk-open</code> button and when it's clicked, <code>run()</code> returns 0.  But double-clicking on a file or selecting a file and pressing Enter doesn't do anything.  As far as I can see, this is because the dialog doesn't know what the default button is in Glade, possibly because the 'Open' button isn't using one of <code>GTK_RESPONSE_ACCEPT</code>, <code>GTK_RESPONSE_OK</code>, <code>GTK_RESPONSE_YES</code> or <code>GTK_RESPONSE_APPLY</code> as its response code; but how do I set the response code to one of these when the button is created through Glade?</p>
  
  <p>I'm using Gtk 3, Python 3 and PyGObject (ie <code>gi.repository</code>).</p>
  
  <p><strong>Edit:</strong> Here is the relevant bit of the Glade file:
  </p>
  
  <pre><code>  &lt;object class="GtkFileChooserDialog" id="file_chooser"&gt;
      &lt;property name="can_focus"&gt;False&lt;/property&gt;
      &lt;property name="modal"&gt;True&lt;/property&gt;
      &lt;property name="type_hint"&gt;dialog&lt;/property&gt;
      &lt;property name="select_multiple"&gt;True&lt;/property&gt;
      &lt;child internal-child="vbox"&gt;
        &lt;object class="GtkBox"&gt;
          &lt;property name="can_focus"&gt;False&lt;/property&gt;
          &lt;property name="orientation"&gt;vertical&lt;/property&gt;
          &lt;property name="spacing"&gt;2&lt;/property&gt;
          &lt;child internal-child="action_area"&gt;
            &lt;object class="GtkButtonBox"&gt;
              &lt;property name="can_focus"&gt;False&lt;/property&gt;
              &lt;property name="layout_style"&gt;end&lt;/property&gt;
              &lt;child&gt;
                &lt;object class="GtkButton" id="button1"&gt;
                  &lt;property name="label"&gt;gtk-cancel&lt;/property&gt;
                  &lt;property name="visible"&gt;True&lt;/property&gt;
                  &lt;property name="can_focus"&gt;True&lt;/property&gt;
                  &lt;property name="receives_default"&gt;True&lt;/property&gt;
                  &lt;property name="use_stock"&gt;True&lt;/property&gt;
                &lt;/object&gt;
                &lt;packing&gt;
                  &lt;property name="expand"&gt;True&lt;/property&gt;
                  &lt;property name="fill"&gt;True&lt;/property&gt;
                  &lt;property name="position"&gt;0&lt;/property&gt;
                &lt;/packing&gt;
              &lt;/child&gt;
              &lt;child&gt;
                &lt;object class="GtkButton" id="button2"&gt;
                  &lt;property name="label"&gt;gtk-open&lt;/property&gt;
                  &lt;property name="visible"&gt;True&lt;/property&gt;
                  &lt;property name="can_focus"&gt;True&lt;/property&gt;
                  &lt;property name="receives_default"&gt;True&lt;/property&gt;
                  &lt;property name="use_stock"&gt;True&lt;/property&gt;
                &lt;/object&gt;
                &lt;packing&gt;
                  &lt;property name="expand"&gt;True&lt;/property&gt;
                  &lt;property name="fill"&gt;True&lt;/property&gt;
                  &lt;property name="position"&gt;1&lt;/property&gt;
                &lt;/packing&gt;
              &lt;/child&gt;
            &lt;/object&gt;
            &lt;packing&gt;
              &lt;property name="expand"&gt;False&lt;/property&gt;
              &lt;property name="fill"&gt;False&lt;/property&gt;
              &lt;property name="position"&gt;0&lt;/property&gt;
            &lt;/packing&gt;
          &lt;/child&gt;
          &lt;child&gt;
            &lt;placeholder/&gt;
          &lt;/child&gt;
        &lt;/object&gt;
      &lt;/child&gt;
      &lt;action-widgets&gt;
        &lt;action-widget response="-1"&gt;button1&lt;/action-widget&gt;
        &lt;action-widget response="-5"&gt;button2&lt;/action-widget&gt;
      &lt;/action-widgets&gt;
    &lt;/object&gt;
  </code></pre>