provide a non-changing category container div at renderCategory()

[haoliangyu]

In short, I am trying to implement a header and a footer in the menu:

-------------------------------
beautiful header
-------------------------------
[ item 1 ] <- highlight
item 2
item 3
-------------------------------
beautiful footer
-------------------------------

The renderCategory() seems to be a solution as it provides a div that I can modify. However, the item list is appended later and this makes it difficult to keep a footer child element at the end.

It would be good (to me) to append the ul element first beforehand. So when I receive the div from the renderCategory() function, the element layout is not changed by the library again and I can feel free to add child elements without worrying their order.