Include performance navigation timing information in browsingContext events

[juliandescottes]

We could include the information from the Performance Navigation Timing interface (https://w3c.github.io/navigation-timing/#dom-performancenavigationtiming) in the various events from the browsingContext module.

network events (from PR #204) contain a timings object filled with as many timings as were available at the event creation time. Similarly here, the browsingContext events could all have a timings object, containing as many timings as possible.