Instead of using string everywhere, have the CSS parser produce enum
values, since they are a lot nicer to work with.
In the future we should generate most of this code based on a list of
supported CSS properties.
LayoutReplaced objects can now participate in inline layout.
It's very hackish, but basically LayoutReplaced will just add itself to
the last line in the containing block.
This patch gets rid of the idea that only LayoutInline subclasses can
be split into lines, by moving the split_into_lines() virtual up to
LayoutNode and overriding it in LayoutReplaced.
This patch adds parsing of <img> into HTMLImageElement objects.
It also adds LayoutImage and its parent class LayoutReplaced, which is
going to represent CSS "replaced elements."
