Wrap LLView::child_list_t in a class AIList.

So far not doing anything. Iterators already keep a pointer to the
container they belong to.
Do not pre- increment the loop iterator in LLView::drawChildren

This is in fact much safer, because the only way to invalidate an
interator in the first place (in this code) is by calling removeChild,
which *already* has an assert never to remove a child that is being
iterated over (by means of the mInDraw flag).
Make AIList resilient against invalidating iterators.

This adds a counter and a 'dead' flag to the data stored in the linked
list. The counter counts the number of iterators (still) pointing to an
element and the dead flag is set to indicate the element was erased when
iterators are still pointing to it. As a result, iterators are NEVER
invalidated. Of course, such elements are subsequentially skipped when
iterating over the list. Assertions protect against dereferencing an
erased iterator, but incrementing or decremention still works: it is
still well-defined what the next (non erased) element is, assuming the
element wasn't erased (yet), but would be erased delayed - or assuming
the iterator would have been incremented (decremented) in advance to
erasing the element.