ENH: more general groupFlipVert()

- used in visual.RatingScale.setFlipVert
- flips all items in a list, preserving their relative positions (grouping)
- could also be used to flip all visual items for a trial:
  visual.helpers.groupFlipVert(allTrialItemsList)
- not checked: setOri(..., '+') should reverse its sense if a stim is flipped