This has several advantages over the current manual demuxing currently
being performed. PlaybackManager hides the specific demuxer being used,
which will allow more codecs to be added transparently to LibWeb. It
also provides buffering and controls playback rate for us.
Further, it will allow us to much more easily implement the "media
timeline" to render a timestamp and implement seeking.
This ports MouseEvent, UIEvent, WheelEvent, and Event to new String.
They all had a dependency to T::create() in
WebDriverConnection::fire_an_event() and therefore had to be ported in
the same commit.
