title: Center buttons in dialog created with "gtk_dialog_new_with_buttons"
body: <p>I am writing a Gtk application and I am having troubles centering some buttons.
  I created a popup window in this way:</p>
  
  <pre><code>dialog = gtk_dialog_new_with_buttons ("Add element",
                                        GTK_WINDOW(gElems-&gt;window),
                                        GTK_DIALOG_DESTROY_WITH_PARENT | GTK_DIALOG_MODAL,
                                        "OK",
                                        GTK_RESPONSE_ACCEPT,
                                        "Cancel",
                                        GTK_RESPONSE_REJECT,
                                        NULL);
  </code></pre>
  
  <p>Everything is ok, except for the OK and Cancel buttons, which are aligned to the right end of the popup window, while I would like them to be centered (as you can see from the picture). </p>
  
  <p><img src="https://i.stack.imgur.com/IwS9x.png" alt="enter image description here"></p>
  
  <p>Is there a way to do so without using additional containers to store the buttons?</p>