Support new color notations and wide gamut colors

Summary:

CSS is moving towards a more homogenous colour notation; rgba(...) is now considered legacy. It would be great if Inkscape could interpret the new notation as well.

Take RGB for example:

/* Syntax with space-separated values */
rgb(255 255 255)
rgb(255 255 255 / .5)

/* Syntax with comma-separated values */
rgb(255, 255, 255)
rgb(255, 255, 255, .5)

/* Legacy rgba() syntax */
rgba(255 255 255)
rgba(255 255 255 / .5)

rgba(255, 255, 255)
rgba(255, 255, 255, .5)

Probably harder to support are the new wide gamut colour spaces. It's probably not as urgent as the notation mentioned above, but still something to keep in mind:

color(display-p3 1 0.5 0);
color(display-p3 1 0.5 0 / .5);

Wide gamut screens are not yet widely available, although all Mac laptops with Apple silicon support Display P3. We'll probably have to wait a few years to have this on Linux as well.