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.

include/knob.h

@@ -148,12 +148,11 @@ private:
 	QPixmap * m_knobPixmap;
 	BoolModel m_volumeKnob;
 
-	float m_mouseOffset;
+	QPoint m_mouseOffset;
 	QPoint m_origMousePos;
+	float m_origValue;
 	bool m_buttonPressed;
 
-	int m_magneticDecay;
-
 	float m_totalAngle;
 	int m_angle;
 	QImage m_cache;