Created an intermediate class for sweeped knobs.

This class handles knobs with sweep: the number of degrees of motion it
has, centered at 12 o'clock. (The equivalent of the total mechanical
travel in a potentiometer.)  Both rotary knobs and center-tapped knobs
are of this type; continuously rotating knobs are not.

Also refactored some code, so that the methods are in the proper order,
and corrected the relevant JavaDoc.
Modified MainActivity

The AbsKnob class now loads an image by default; no need to set the
image in MainActivity.