Rewind in debug controls

Currently, the speed options for debug mode go from 0x (paused) to 5x. It would be nice to have a rewind option, as well.

The implementation would be trivial for animations, where you just want to subtract from the current time every frame. For non-debug simulations, this is impossible anyway, so the slider should only go to zero.

Debugged simulations and interactions are the hard case. We know the past states, but we don't know their times. If we did know the times, this would be doable, but complex. (The new debug state type would have to contain two lists of paired state-and-timestamp, and a current timestamp: `([(a, Double)], [(a, Double)], Double)` This also opens the question of whether you want the history slider scaled by frame, or by time.