[dev-context] derived colors (mkiv)

Hi, there is currently no bookkeeping ('name-->id') for derived colors in mkiv; i.e. \definecolor[foo] [black] does not register 'foo'. \csname(ca:foo)\endcsname is the only way to access the color id. While this in no problem in TeX code, using this in lua is not very handy. I tried ugly stuff like c_id = format("\\csname(ca:%s)\\endcsname",c) but when I compare 'c_id' with the id of black, the values '1' and 1 (id of black) differ when compared. Mhhhh. I think that the best way to handle derived colors properly is to add their name to the attributes list too. It's only a small patch and it shouldn't do any harm. Best wishes, Peter