Add speed up/slow down controls by percentage

[lathe26]

This is a feature request: Add speed up/slow down controls by percentage.
For example, change the speed by 200% means to divide all the "Delay" values by 1/2. This can be applied to all frames or the frames that have been selected. Currently, when there are a lot of frames, it can be a pain to manually edit every delay value one at a time.

[Aivars]

That's a good idea, I'll add it to suggestions list.