WindowHandling: refactor screenshot handling/remember scroll position

On long pages the user scrolls down to a button/link to navigate away from the current site. For the user this page seems to instantly jump to the top, since this is the cached initial state, which is now shown in the window handler intermediate page.

Please store the html (and it's resources) not during initial site load, but the most recent version of it, just when the user clicks on buttons or links.