Portal.view.blog.List: lazy-loading for images

@rwaters: while this one would be resolved with a buffered renderer / buffered store out of the box, we should add something now.

my idea is to initially not define the background-image url for each item and add an IntersectionObserver. Adding URLs while scrolling with a new config like preloadImages {Number} to add (pre-)load a couple more.

I will give it a try.