Need way to drive Animations explicitly, not over set duration only

There needs to be a way to drive an animation manually, rather than
over a set time. This would be useful e.g. when the animation is
attached to a swipe gesture and the animation should follow the
progression of the finger rather than just be triggered and run for
a fixed time.

A new "currentProgress" (or "progression") property with a value
between 0..1 makes more sense than something relative to the total
duration time of the animation. It would need to work for ParallelAnimation
and SequentialAnimation too, and support not only strict forward
progression of the animation but also backwards, as well as "continuing"
the animation on the duration timer.