Add Option to Display Top Level page in BookNavigationBlock

If the BookNavigationBlock does not have the configuration option "all pages", it only displays the subpages in the navigation block.

On line 143 there is this code:

          $data = $this->bookManager->bookTreeAllData($node->book['bid'], $node->book);
          $book_menus[$book_id] = $this->bookManager->bookTreeOutput($data);

which outputs the items in the book using ->bookTreeOutput()

but for some reason, on line 177, it only outputs the subpages:

 $tree = $this->bookManager->bookTreeAllData($node->book['bid'], $node->book);

        // There should only be one element at the top level.
        $data = array_shift($tree);
        $below = $this->bookManager->bookTreeOutput($data['below']);
        if (!empty($below)) {
          return $below;
        }

I guess some developers would only want the some pages, so I'm not sure if this is a bug, or should be an option.