Empty navigation means should be unavailable

[gautierchomel]

The Navigation panel starts with a dropdown menu to select a navigation mean (TOC, landmark, gotopage, annotation, bookmark). When one of those means have no entry, it is still reachable and present an empty panel.

To prevent users from searching to understand where they are, when a navigation mean is empty, it should not be reachable.

[gautierchomel]

Note that this is the half the case when that panel is not docked (landmark is disabled but not got to page)