Refactor of the ERXNavigation package out of ERExtensions to its own project

It seemed to me that this package was pretty much completely isolated from the rest of ERExtensions. This package depended on ERExtensions but nothing else in ERExtensions depended on it. I cannot think of an easier case for re-factoring.

And if anyone wants to suggest that the ERExtensions project _should_ contain absolutely everything that everyone would ever need, please explain. But I figure that java has classpaths for a reason. And separating things that are different and not dependent on each other seems a reasonable goal.