I also changed a bit the new color serialization by adding a (color …)
symbol framing the contents, for cases where we don't have a specific
property name, e.g. for the color history list stored in colorrc, unlike
for GimpConfig GeglColor properties.

While doing this, I moved GeglColor property deserialization code into
gimp_scanner_parse_color() which is now able to recognize both older
(color-rgb|rgba|hsv|hsva with no color space) and newer serialization
formats ("color", color model agnostic and space aware).