Provide methods to access custom built menu model

[krichter722]

When menu models are built in the backing bean rather than declaring them in XHTML, one can't assign an id (DefaultMenuItem.id is used internally only and doesn't refer to the id attribute) or clientId. Therefore it's hard to click them through Selenium. If the menu is nested the click target needs to be identified by an address consisting of the index or label (or else) of the parent menus to click before the nested submenus become visible.

Besides the ability to click on a menu item it'd be helpful to have a method to investigate the menu structure, e.g. check that the submenu under the second menu item has 3 items. This could be done by clicking all three of them through Selenium, however compression in one method would be nice.