Move Tweener.slowDownFactor into St

It has probably crossed the line to evil by a mile or so, but here
it is: a Tweener.slowDownFactor replacement used by all animations
without exception.
While at it, update Tweener to use the new setTimeScale() upstream
function instead of adjusting the timeline directly.

https://bugzilla.gnome.org/show_bug.cgi?id=622249