[Request] Symmetrize finger poses

[fluffy-critter]

When I edit my hand gesture poses I often wish that I had a way of editing both hands simultaneously. It would be really nice if the pose editor had a checkbox for "keep symmetry" and maybe buttons for "transfer L->R" and "transfer R->L" or the like.

There might also be a use case for that on other body parts as well, but hand gestures in particular very often should be the same on both hands.