While the spec says that `accentcolor` and `accentcolortext` resolve
relative to the `accent-color` property this isn't implemented by any
other browsers.
This effectively reverts d04b745 but doesn't change behavior since that
was already done in 92897a1.
These get computed to an equivalent RGBColorStyleValue.
To support this, we now store the computed color-scheme on the
ComputationContext when computing properties that might contain a color.
This has a nice bonus of correcting the css-accent-color test's result.
Previously, some StyleValues created a large number of intermediate
strings during serialization. Passing a StringBUilder into the
serialization function allows us to avoid a large number of these
unnecessary allocations.
For us, that's KeywordStyleValue and CustomIdentStyleValue.
CustomIdentStyleValue now has a .cpp file to reduce the number of
includes in its header file.
