Use CSS instead of JavaScript for overlay and to reference images (close button, loading status)

The changes result in using CSS only for the overlay so that it stays at the correct position even when scrolling large images. Previously the user could scroll the overlay away.

JavaScript does not longer generate references to images (close button, loading status) but uses CSS classes instead. That allows to exchange the images if desired with the site's CSS and the default images are given relative to the CSS file, too. Therefore Lightbox 2 can now be extracted into any subfolder without the need to have an images folder in the base directory of the page including Lightbox 2.