Customization of touch input line & more

- To avoid confusion with the vertical reference lines, the line
displayed when the user touches the graph is now called "touch input
line" (was called vertical line before).
- Added property colorTouchInputLine to customize the color of the
touch input line
- Added property alphaTouchInputLine to customize the alpha of the
touch input line
- Added property widthTouchInputLine to customize the width of the
touch input line
- New method distanceToClosestPoint that calculates the distance
between the closest point and the user's touch input
- New method setColorTouchInputLine to dynamically set the color of the
touch input line