Animating NLA Strip's playback speed

VSE has a new retiming tool which allows to animate the speed of a clip using keyframes.

NLA strips should get a similar tool as well, to allow e.g. smoothly changing the speed of a cyclic animation over time. This might fit as part of the Animation datablock project.

Currently this must be done with "Animated Strip Time", but for this you must animate the frame number and not a playback scale. This makes it much harder to do a smooth transition from 1x to 2x playback speed.