Changed int progress to float level

In "real" applications, knobs are going to be used for things like
volume and pan, which should have float values associated with them.

The "progress" field was only implemented because I was trying to
imitate a SeekBar. I no longer thing this is a good idea.
Refactoring

Any knob with max and min values cannot be continuous, so I am
refactoring out the abstract classes.
Deleted AbsKnob

It is now just Knob; it is no longer abstract.