BListView: Fix missing update for list items.
- According to the Be Book, one of BListItem::Update()'s responsibilities
is updating the item's internal height/width. However, while we were
providing such updates in the case of the item initially being added
to the view, or when the list view's font was changed, we were not doing
so on frame resize. This would lead to items having a stale width field
in such a circumstance, leading to possibly incorrect behavior in apps
relying on that field to be correct.