Add a knob to the widget

Allows to optionally have a knob above the stroke.

Configuration : 
- ```displayKnob``` : boolean to display the knob or not. Default to false to be backward compatible
- ```knobColor``` : (optionnal) background color of the knob, default to ```fgColor```
- ```knobRadius``` : (required) must be expressed as a fractino of the ```lineWidth```. Default to ```0``` in order to avoid having special cases further in the code.

Notes : It reduces the whole widget radius if activated, or the knob would overflow the canvas.