Now makes use of Tossable as per the modal and interactive push back transition demos. This pattern has now been proven in three different transitions and can likely be formalized.
Draggable might create its own gesture recognizer when it gets add'd. If we want to coordinate motion off of that gesture recognizer we need some way to access it. This was the simplest stop-gap solution I could come up with.
An alternative solution that I'd considered was allowing runtime.add to return a generic type. In this case Draggable could return a pan gesture recognizer. Happy to explore this in the future.