Add method in Router to run callback after before leave confirmation.

[TatuLund]

Describe your motivation

In Vaadin 8 it is possible to run call back after before leave confirmed. This is handy for example with in case of logout button. See code example below.

        var logout = new MenuBar();
        logout.setId("logout");
        var item = logout.addItem(getTranslation(LOGOUT), e -> {
            // Use runAfterLeaveConfirmation wrapper to run the logout in based
            // on beforeLeave of the current view. 
            nav.runAfterLeaveConfirmation(() -> {
                logger.info("User '{}' logout",
                        CurrentUser.get().get().getName());
                ui.getSession().getSession().invalidate();
                ui.getPage().reload();
            });
        });

Describe the solution you'd like

It would be preferable a method with similar naming in convenient place. This would help migrating Vaadin 8 application using this API.

Additional context

In addition to helping the migration from Vaadin 8, the method could be useful as is.