Regenerates the normal appearance of every widget from the field's current selection, as an interactive processor does for variable text.
public void GenerateAppearances()
ISO 32000-2 §12.7.4.3 and §12.7.5.4. Each widget gets a new form XObject as its normal appearance, built as for a text field
(GenerateAppearances(): MK background and border, default appearance font, size and colour,
quadding): a combo box shows the selected option's displayed text (or an editable one's typed value) on one line; a list box
shows its options one per line from the top index (TopIndex), an auto size being 12 points, with a highlight
behind each selected option.