Add class extension to hande color encoding.

git-svn-id: svn+ssh://svn.savannah.nongnu.org/gap/trunk@2751 d25b4930-3292-e311-b074-0010c6de5da8
Switch to color extension instead of relying on gnustep internal colorFromString.

git-svn-id: svn+ssh://svn.savannah.nongnu.org/gap/trunk@2752 d25b4930-3292-e311-b074-0010c6de5da8