Instead of making the developer do the casting, we should do this.  Also, the C
calls to these functions return the correct type:

static GtkWidget *
create_menu_item (GtkAction *action)
{
  GType menu_item_type;

  // this type ends up being GTK_TYPE_IMAGE_MENU_ITEM
  menu_item_type = GTK_ACTION_GET_CLASS (action)->menu_item_type;

  return g_object_new (menu_item_type, NULL);
}

static GtkWidget *
create_tool_item (GtkAction *action)
{
  GType toolbar_item_type;

  // this type ends up being GTK_TYPE_TOOL_BUTTON
  toolbar_item_type = GTK_ACTION_GET_CLASS (action)->toolbar_item_type;

  return g_object_new (toolbar_item_type, NULL);
}