CSSJanus: Fix handling of CSS3 color syntaxes

The 'color' rule, and by extension 'four_notation_color' too,
only understood #rrggbb and named colors. Extend it to match
rgb[a](...) and hsl[a](...) syntaxes as well.

This makes usage of rgb(a)/hsl(a) syntax in declarations like
'border-color: a b c d;' be interpreted and flipped correctly.

Change-Id: I218a9aa55a86b3d955b92375c1a209fdde312138