Rename OnPlayHeadChangedEvent to OnPlayHeadChanged
Refactor VideoLoaded Event in EngineView

VideoLoaded now uses the EventHandler delegate, added in C# 2.0, as
opposed to a custom event delegate.

Adds race condition prevention EngineView.OnVideoLoaded.
Add CaptionTimestampChanged event to Timeline

Adds the CaptionTimestampChanged event to Timeline, which is invoked
everytime one or more Caption Timestamp (Begin, End, or Duration) are
modified in the timeline.

The event is handled by the main form so that it can update the
CaptionView's displayed value to the current value.