This patch makes StyleProperties heap-allocated and ref-counted so that
a LayoutNode can be without one. The ref-counting also allows anonymous
blocks to share style with their parent block.
LayoutText never needs a StyleProperties, since text always inherits
style from its parent element. This is handled by style_properties().
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.
