Let's start moving away from using raw strings for CSS identifiers.
The idea here is to use IdentifierStyleValue with a CSS::ValueID inside
for all CSS identifier values.
This is definitely not fully-featured, but basically we now handle
the clear property by forcing the cleared box below the bottom-most
floated box on the relevant side.
Instead of doing a CSS property lookup for the line style of each
border edge during paint, we now cache the final CSS::LineStyle to use
in the Layout::BorderData.
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.
This patch also adds the ability for Length to contain percentage
values. This is a little off-spec, but will make storing and dealing
with lengths a lot easier.
To resolve a Length to a px-or-auto Length, there are now helpers
for that. After calling them, you no longer have to think about
em, rem, %, and such things.
StyleProperties is really only the specified "input" to what eventually
becomes the used/computed style we use for layout and painting.
Unlike StyleProperties, LayoutStyle will have strongly typed values for
everything it contains (i.e no CSS::ValueID or strings, etc.)
This first patch moves z-index into LayoutStyle.
