Made knob abstract.

In this way, different subclasses can be different types of knobs:
rotary knobs, centered (panned) knobs, continuous rotary knobs, etc.
Finished separating concrete and abstract knobs.

AbsKnob is now a bona fide abstract base class. The concrete subclass is
RotaryKnob. Next up is to do a knob that is centered (like a pan knob).