We try scrolling a Node with the handle_mousewheel event, but if it
isn't scrollable, the event should be passed back up to the page
host. This is the first step in that process.
This is rather crude, but you can now use the mouse wheel to scroll up
and down in block-level boxes with clipped overflowing content.
There's no limit to how far you can scroll in either direction, since
we don't yet track how much overflow there is. But it's a start. :^)
The generic is<T>() uses dynamic_cast which is fine in the majority
of cases, but when one of them shows up in profiles, we can make it
faster by answering the is-a question manually.
