UTF-8 BOX mode: Allow filling of boxes

[dmullis]

1. Add a sweep-line iterator to identify closed boxes
2. Modify SVG output generator to emit a single <path> element for each
3. Add a name e.g.box to the CSS class attribute of each
4. If text is contained within a box, derive a short string from it that is legal as a CSS class name, and add that name to the class attribute of the <path> element generated for that box.