Defines the possible options and option groups for a <select> [X]HTML element.
Primarily used by the org.apache.tapestry5.corelib.components.Select component, but potentially used by
anything similar, that needs to present a list of options to the user. Generally paired with a org.apache.tapestry5.ValueEncoder to create client-side representations of server-side values.
The list of ungrouped options, which appear after any grouped options. Generally, a model will either provide
option groups or ungrouped options, but not both.