The previous implementation was essentially rewriting Tossable. This change refactors the interaction to make use of the Tossable interaction by providing a TransitionSpring to Tossable. This takes advantage of the fact that TransitionSpring is a subclass of Spring and that Tossable can be provided with an existing Spring instance.
Note that this diff makes it possible to drag the modal dialog on the x-axis. In a follow-up diff I will introduce constraints so that this interaction can be locked to the y axis.