AFBClient: implement event handler dispatch

AFBClient: implement event handler dispatch

* requestSurface will store label if successful.
* onEvent calls AFBClient::Impl::event to dispatch the event.
* stripped down onCall and onEvent implementations.
* event names from onEvent are matched on-the-fly to
  AFBClient::EventType.
* requestSurface with a name that is already known will fail
  immediately.
* Removal of event handler not supported.
* Removal of surfaces not supported - assume the application
  lives only as long as it's surface.
* Add minimal Event_Active event handler to main.

Signed-off-by: Marcus Fritzsch <marcus_fritzsch@mentor.com>