The solution is to provide a standard implementation in the form of a mix-in,
which directly houses a `ZoomWindow` instance. Moreover, the latter
is deemed a prominent use case for the time::Control, allowing other
components to attach and push changes of the zoom state or register
as listeners to react to state changes.
Actually, the `TimelineLayout`, which hosts all the actual visible
widgets forming the timeline-UI, now integrates this mix-in; and since
`TimelineLayout` is passed to `TimelineController` and used there as
reference-`CanvasHook` for the root track, this implementation of
the `DisplayMetric` interface will ''effectively be used by all
widgets'' attached to the timeline canvas.
