Retire TimedItem events

Retire the long-deprecated feature TimedItem events (start, iteration, end, cancel).

    Retiring TimedItem events involves retiring
      TimedItem.addEventListener
      TimedItem.removeEventListener
      TimedItem._generateChildLegacyEventsForRange
      TimedItem._generateLegacyEvents
      TimedItem._hasLegacyEventHandlers
      AnimationPlayer._checkForLegacyHandlers()
      AnimationPlayer._legacyHandlerAdded()
      AnimationPlayer._needsLegacyHandlerPass
    and retiring the tests
      impl-test-player-sees-handlers.html
      test-timeditem-addeventlistener.html
      test-timeditem-events.html
Merge pull request #623 from ericwilligers/retire-timed-item-events

Retire TimedItem events