This will be needed by the layout node, which may change what is painted
when the position of the frame image is not the same as the element's
current time.
The on_end_of_stream callback was added to notify clients that video
playback has stopped when we didn't have a way to retrieve the playback
state from Video::PlaybackManager. Now that we do, we should consolidate
on using the on_playback_state_change callback to detect such changes.
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.
