Retimer: Playback speed

[Laserschwert]

Would it be possible to add the option to retime the sim via a playback speed factor, instead of animating the frame by hand? Both ways have their uses, I think.

[tyFlow]

I don't think it's out of the question to eventually add a simple playback speed option, however I added the spinner/curve method for extra control.

I will add a simple playback speed value to the todo list, since I think it would still be useful.