A list of custom navigation state that was returned by listeners for
the saveNavigation event. This state is passed as an argument
for the loadNavigation. This mechanism can be used by extension to
implement custom state saving.
The names of all pages that were visible at the point in time when the navigation state was saved. Note: Usually only one page is visible at a time, but it's possible for more than one page to be visible, e.g. in the inbox.
Navigation state saved by XUtil.saveNavigation and loaded by XUtil.loadNavigation.