Added overdraw visualization.

Use a native object instead in cases like these, and when the object is static like this do it as a literal (`triggerVisualizer('highlight', {index: it.getIndex()})`).

As for this argument, It'd be cleaner to make the trigger call be explicit about what context it is operating on. For example, having an `applyToStep(opt_index)` (where omitting does it to the current step), `applyToFrame(opt_frameNumber)`, etc makes it easier to reason about for people using the Playback API, removes the potential bugs (like forgetting to set the index arg - or knowing that the highlight visualizer even needs it), etc.