Add option for showing controlArrow on horizontal slides

Added option, default behaviour is still to show the arrows. Since keyboard navigation depended on simulating clicks on the controlArrows (why?), created new (wrapper) methods for moving left and right, and a function doing the actual work, mostly copied from the controlArrow onClick behaviour, with some extra steps and checks.

Please note that I'm a novice/hobby coder, so I'm bound to have mode some style errors at the least. I do feel that keyboard navigation should not depend on the arrows being there (and visible), and that there should be an easy way of not having arrows.