"div p" now generates a Selector with two components where the second
component is a type=TagName, value="p", relation=Descendant.
We still don't handle matching of these, but at least we parse them.
The default style for "a" tags now has { color: -libhtml-link; }.
We implement this vendor-specific property by querying the containing
document for the appropriate link color.
Currently we only use the basic link color, but in the future this can
be extended to remember visited links, etc.
The parser now kinda recognizes immediate child selectors, !important,
and various other little things.
It's still extremely far from robust and/or correct. :^)
Also remove the color values from the ComputedStyle object and get them
via StyleProperties instead.
At the moment, we only handle colors that Color::from_string() parses.
