This change adds support for the `colorspace` attribute on color inputs
and implements serialization of the input value accordingly.
The color picker from servoshell doesn't yet support non-rgb colorspaces
(and neither does the embedding API), so we convert to and from rgb when
communicating with the embedder.
Testing: New tests start to pass
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Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>
