Javadoc
A JHelp can have a number of navigators.
One of navigators is active.
How they are presented depends on the UI, but they may be a collection
of tabs, with the active tab being at the front.
Each navigator listens to changes to the HelpModel.
A navigator can also tell the model to change--the change
is propagated to the other navigators, this component, and
the content viewer if they all use the same
HelpModel instance.