LibWeb keeps growing and the Web namespace is filling up fast.
Let's put DOM stuff into Web::DOM, just like we already started doing
with SVG stuff in Web::SVG.
To make this possible, I also had to give each LayoutNode a Document&
so it can resolve document-specific colors correctly. There's probably
ways to avoid having this extra member by resolving colors later, but
this works for now.
Table row layout is now split into two phases:
1. Compute all the column widths (even taking colspan into account!)
2. Place all cells at the correct x,y offsets based on column widths.
Both phases visit all rows and all cells.
