Selects the on state value, or nothing with Off: sets the field's value (V) and the appearance
state (AS) of every widget that has appearance states.
public void SetValue(CosName value)
An on state of one of the widgets (OnStateNames), or Off.
ISO 32000-2 §12.7.5.2.3, §12.7.5.2.4 and Table 229. V is stored as a name in the field's own dictionary and each
widget's AS is set to its on state when it turns on, else to Off, so that both keys agree (the clauses' "shall
also be"). Every check box widget with the on state turns on (duplicated boxes toggle together, §12.7.5.2.3 example 2). Of radio
buttons the first widget with the on state turns on; with RadiosInUnison every widget with that on state, or with the same
Opt export value, turns on with it. The existing appearances are kept: GenerateAppearances() makes new ones.
Read-only fields are set too: the flag restricts the user, not the program. When the form also has an XFA resource (Annex K),
only the AcroForm value changes and an XfaDataNotUpdated information diagnostic is recorded.