We now keep the color value as a StyleValue up until we go to paint the
gradient, which makes `currentColor` work, along with any other color
values that can't be immediately converted into a `Gfx::Color` while
parsing.
This solves an awkward dependency cycle, where CalculatedStyleValue
needs the definition of Percentage, but including that would also pull
in PercentageOr, which in turn needs CalculatedStyleValue.
Many places that previously included StyleValue.h no longer need to. :^)
