Extract arrowsTogether method and other cleanup (REnamer, 4ed8742)

Things are slightly neater with the code that determines whether the scroll arrows are together or at opposite ends of the scroller is extracted out into a separate method.

By way of other cleanup, we also rename the knob variable to knobRect for consistency with the other variables in the rectForPart: method.

Signed-off-by: Greg Hurrell <greg@hurrell.net>