Prevent focused items from receiving click events

We ran into a situation where users were clicking on elements such as buttons and links which were the focus of a step.  This was causing problems since the page might be change or even navigating while the user is half way through the guide.

This PR adds another `div` at a higher z-index than the `introjs-helperLayer` and also attaches a click handler to block clicks from bubbling down.  The new `div` follows the `introjs-helperLayer` `div` as its resized when the guide progresses.