Merge Icinga\Web\MenuItem into Icinga\Web\Menu

We do not need an additional class for the menu's root item just to
encapsulate our config handling, so these two classes were now merged
into a single one. Icinga\Web\Menu now also extends from
RecursiveIterator to be able to render the menu more efficient to html.

refs #6153
Adjust the test for Icinga\Web\MenuItem as its test subject has changed

refs #6153
Add a HTML renderer for Icinga\Web\Menu

The menu was being rendered through recursive partials before. The
overhead this fact implicates is not as efficient as standard recursion
so there is now a special renderer for Icinga\Web\Menu utilizing
the RecursiveIteratorIterator

refs #6153
Merge branch 'bugfix/activate-menu-entry-based-on-the-url-6153'

fixes #6153