Move some things around. Move the duration of the current cycle
to the clock. Also add the estimated next timeout to the clock.
Add a generic media specific counter to the clock.

Clean up the position_bar info. We can do with only a double beat
value and make the signature in floats.

Flesh out the io_position info. This has now the information needed
to convert a raw clock time into a stream time. It basically has
the same kind of features as GStreamer segments such as looping,
variable rate playback etc.. It also contains the state of the
timeline (paused/playing) and it can be used to update the position
and state from clients.

There is also extended information in the position field that
clients can update when they can.

Plugins basically only update the clock info they get (and use
the position info to check if they are slaved or not).

Before each cycle, check if there is a pending position update and
apply it.