I've created a "Scrubber" component that's meant to control a TimelineLite instance. The timeline is passed to other components so that they can add tweens, timelines, etc. However, these may not be added synchronously. The problem, then, is that I have no way of updating my Scrubber when the duration of the timeline changes.   On a related note, timelines only one of each callback type (onUpdate, etc.).   Given these two issues (duration not observable, no "safe" event listener adding), is