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.
We don't support incremental relayout of subtrees (only single nodes)
but let's not crash the browser just because this happens. We can keep
the browser up and just complain in the debug log instead.
There's more to life than inline-vs-block, so we have to take all the
non-block non-inline display types into account when computing whether
a block should say children_are_inline() == true.
