Gets the indices in Options of the selected items, ascending: each value matched against the options' export
values, then their displayed text. I decides between options with the same text when it agrees with the value.
public IReadOnlyList<int> SelectedIndices { get; }
ISO 32000-2 §12.7.5.4, Table 234: when I differs from V, "the V entry shall be used". A value no option has records
ChoiceValueNotInOptions, except in an editable combo box, where any text is legal.