Gets the selected on state (V, inheritable); Off when the value is absent or nothing is selected.
public CosName Value { get; }
ISO 32000-2 §12.7.5.2.3 and §12.7.5.2.4 (default Off). A string reads as the name it spells with a
FieldValueTypeInvalid diagnostic; a name no widget has as its on state records ButtonValueUnknownState and is
returned as stored.