Since the beginning, there was in getCategoryChildItemList Renderer
API the possibility to pass a parameter 'current_category'. The idea
was to allow displaying a category which is not part of what the item
list will return (typically, if you expire a category, and if you check
permission, the category will not be in item list any more).

But the way it was implemented was too inconvenient and it looks never
used. So improve several things:
- possibility to pass multiple categories for multi value cases
- reuse renderer to render current categories instead of just reinjecting
  the url (which is not user friendly)
- implement handling of current_category_list in getCategoryChildItemList
  to allow caching the result of the Renderer. The only part which is not
  cached is the rendering of current categories.

/reviewed-on !201