UI: smoother knobs

This updates knobs to feel smoother and allow the user to have finer
control over the value of the knob by multiplying the mouse drag delta
by a smoothness factor.

Also since it is significantly easier to point the knob where you want
it, I removed the magnet effect. This can easily be put back in; try it
out and see what you think.

If we like this change, then it should probably be done for faders and
lcd spinboxes as well.