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. :^)
Also stop exposing the DOM cursor as a mutable reference on Frame,
since event handling code was using that to mess with the text offset
directly. Setting the cursor now always goes through the Frame where
we can reset the blink cycle appropriately.
This makes cursor movement look a lot more natural. :^)
