decisionElements

Every DecisionElement in this model, in construction order (§4.8.2).

The counterpart of ksl.animation.animatableModelElements(), and written the same way over the same source, because the problem is the same: a layer added after the model was built has to be able to find what it attaches to. Model.getModelElements() is internal, so this lives in KSLCore — as animation's does.