Spotlight: Show a spotlight over a Web page

This class can show a spotlight over a Web page.

It can create a transparent canvas on the top of the Web page with a dark area and a light rounded spot area that make the content below it appear as if it highlighted by a spotlight.

The spotlight can be animated so it can move at a given speed.

Callback functions may be invoked when the user drags the mouse over or clicks in the the spotlight or the canvas area, as well when the animation starts and ends.