Gets the header cells the cell's Headers attribute names by their element IDs, in the order given; empty when it names none.
public IReadOnlyList<LayoutTableCell> Headers { get; }
ISO 32000-2 Table 384 (Headers) and §14.8.4.8.3. IDs that name no header cell of the tree are skipped. Headers found by
the implicit algorithm of §14.8.4.8.3 (from Scope and position) are not added.