Follow: handle view scrolling nicely

[aidanholm]

Currently, scrolling a webview with visible hints causes the hints to scroll out of view, but no new hints are generated.

It'd be nice to have hints shown for links, as they scroll into view. One approach would be to group target elements by y-index into buckets of ~50px when entering follow mode; from there, it's easy to re-render (#292) the visible buckets of elements on scroll (with appropriate de-bouncing). Obvious problem: what should be done if the user partially filters the visible hints via keyboard, and then scrolls? Should the user filter be cleared if the currently focused hint is scrolled out of view? Should the filter be continually re-applied as elements scroll into view? If so, filtering to a single item in such an indirect fashion shouldn't trigger hint activation.

Probably a wishlist item at the moment, but it'd be pretty nice to have.