Make topmenu HTML editable by the event page_block_html_topmenu_gethtml_after

Currently, the `page_block_html_topmenu_gethtml_after` event is dispatched like that:

    $html = $this->_getHtml($this->_menu, $childrenWrapClass);
    Mage::dispatchEvent('page_block_html_topmenu_gethtml_after', array(
        'menu' => $this->_menu,
        'html' => $html
    ));

Since `$html` is a string, it is not possible to alter it. But I think it should definitely be possible to alter it! Hence I wrapped it into a transport object like in the `core_block_abstract_to_html_after` event.

This pull request is based on a [Magento Stackexchange question](http://magento.stackexchange.com/questions/634/how-can-i-alter-a-string-passed-by-an-event/644).