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.
Since LayoutText inherits all of its style information from its parent
Element anyway, it makes more sense to load the font at a higher level.
And since the font depends only on the style and nothing else, this
patch moves font loading (and caching) into StyleProperties. This could
be made a lot smarter to avoid loading the same font many times, etc.
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.
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.
