the order in which BeforeEnterObservers, BeforeLeaveObservers are executed should be determinable

[berndhopp]

right now, it is impossible to say in which order Observers are executed, which is a problem for authentication. An authentication-observer should be the first observer to be executed, because otherwise other observers should probably not be executed at all, when the authentication-observer did not pass.
So my suggestion is to always make sure that the manually added observers ( per UI#addBeforeEnterListener() ) are the first to be executed, and to add a new method 'UI#setBeforeEnterListenerComparator(Comparator)'. So when a comparator is provided, the order of execution of the observers is always determined by the comparator.

[pleku]

Any concerns for implementing this as proposed, @Legioth @caalador ?

[Legioth]

@pleku There's already a PR with a more promising implementation approach.