Add the option to define custom renderers for menu items and implement renderers to
display all unhandled services and host in a badge in the navigation.
resolves#7114
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
Thomas Gelf
Eric Lippmann
Matthias Jentsch
Johannes Meyer