ISIS-550: Oscar's work to date on ActionInvokedEvent

- Refactored ActionInvocationFacetViaMethod (distinguish meaning of null, introduced internalInvoke)
- New PostsActionInvokedEvent annotation class
- New facet interface, abstract class, implementation (latter subclassing ActionInvocationFacetViaMethod).
ISIS-550: minor refactoring of InvocationResult

- make nested static class
- use factory methods to avoid boolean in method signature.