That's what this class really is; in fact that's what the first line of
the comment says it is.
This commit does not rename the main files, since those will contain
other time-related classes in a little bit.
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.
