Gets the outline's top-level items, each with its children.
public IReadOnlyList<PdfOutlineItem> Items { get; }
ISO 32000-2 §12.3.3. The items as the outline's links are now: the outline is walked again when a dictionary the last walk read has changed (through this API or straight in the COS objects), else the last walk is reused and reading allocates nothing. The items are live views; an item stays the same view while it stays in the outline.