CodeArea now just a Region. Massive simplification!

All the Control baggage was far more trouble than it was worth. Now we're just
a Region, we plug into the Node-based layout stuffs, and we manage our scroll
offset and everything else ourselves. Simpler, more robust, and probably a fair
bit more efficient to boot.
Wired up next/prev-paragraph.

It's shocking how "right" the editor feels now that I can navigate around files
via a combination of next/prev-page and next/prev-paragraph.