Custom offset for Flip Animation

[Cubouse]

Detailed description of your suggestion

There currently is an option to offset by half the animation length.

It would be useful to have a ratio of the animation length instead, or a numerical field for the offset in seconds to create things like run or gallop cycles of 4 legged creatures, where the offset from one side to the other is not half the animation length.

It's already possible to leave an animation playing in the background while the dialog is on top, so it would be nice to be able to preview the animation offset before pressing confirm (when the animation is paused too for that matter).