Bug 596428 - GtkAssistant: Support ending with a progress page

Bug 596428 - GtkAssistant: Support ending with a progress page

- Add gtk_assistant_commit()

  This function discards the visited pages list so the back button is not
  shown on the current page, and removes the cancel button from subsequent
  pages.  Use this when information provided thus far cannot be revisited.

- Don't show the Forward button on a GTK_ASSISTANT_PAGE_PROGRESS if it's
  the last page (according to the forward page function).

- Append a progress page to the GtkAssistant demo.