Implemented by all controls similar to NavigationBar that are interested in
knowing which iterator is currently in focus in a PanelBinding, so that
they can update their display of currency, etc. based on the
current iterator's position.
This event is generated by PanelBinding when there is a change in
the iterator binding which currently has keyboard focus. This allows
controls like NavigationBar, which are displaying the current iterator state
in a Panel, to update their display with the new iterator's settings.