Refactored Navigation #59

Here's my initial proposal, see if this could be improved further.
That's a tricky one. The arguments to build class="" come from Core\Navigation\Core implementation and it wouldn't make sense to process them in Engine\Navigation\Item
unless there is Core\Navigation\Item derived from it to handle this logic.
I wanted to avoid inheritance in Item to make it small as there may be many of these on single page. It seems that there is no other way though