The IFocusManagerGroup interface defines the interface that
any component must implement if it is grouped in sets,
where only one member of the set can be selected at any given time.
For example, a RadioButton implements IFocusManagerGroup
because a set of RadioButtons in the same group
can only have one RadioButton selected at any one time,
and the FocusManager will make sure not to give focus to the RadioButtons
that are not selected in response to moving focus via the Tab key.