ISIS-948, ISIS-947: refactoring event bus, removing special case handling of event bus in Isis core runtime; adding in ability to inject into request-scoped services and for request-scoped services to be event bus subscribers.

Some of the new reference counting logic has moved from EventBusDefault into EventBusService (abstract class in applib).

Also (for ISIS-947) includes enhancements to ServiceInstantiator so that can call toString(), equals() and hashCode() even if there is no session.

In addition:
- ToDoItemSubscriptions (for todoapp) moved to menu item, rather than contributed (better intent traded against slightly worse workflow during demos).
ISIS-950: suppress stack trace in Wicket's error page if exception is recognized.