Changes

Currently a lot of the design is done but subject to change. CSS block/inline layout code and XUL box layout code have been converted to the new design and work well enough that the Firefox UI basically works (although with some glitches). However, <select> form controls and MathML have not yet been converted to the new design, and are disabled on the branch. Some pieces of the table code have been removed, and not yet entirely rewritten: table-layout:fixed is completely broken.