ISIS-948: refactoring the EventBusService (and EventBusServiceDefault subclass) so that the guava EventBus is actually instantiated only once, in effect with singleton scope, rather than with request-scoped.

Although this design is not symmetrical (in the various @PostConstruct and @PreDestroy lifecycle methods), it has many fewer moving parts.  Making the guava EventBus a singleton means that all events flow through this single instance.  We rely on the proxies of the request-scoped services to dispatch to the appropriate instance on the thread (assuming that a transaction is running for that thread).
ISIS-948: adding validation to EventBusService#register method (in EventBusServiceDefault subclass, in fact); improvements to javadoc.
ISIS-537: updates to javadoc only,
ISIS-537: removing unused/no-op code.
ISIS-537: fixing a bug with ModelAbstract#clearHint, adding a unit test to cover.
ISIS-928: removing very old commented out code in JavassistImposteriser.
ISIS-951: introducing menu separators; also getting rid of the btn-warning for prototype, using subtler italics instead.