rgb.txt parsing

[Jean-Marc Lasgouttes]

Angus,

I have commited code to parse the rgb.txt file with lyxlex. Could you
please check that I did it right? The functionality should be the same
as it was.

I did not touch the code which filters the list of colors, but I am
not sure what it does. For example, do we really need to filter out
gray00 and gray100 (especially since it does not filter grey0 and
gray0 which are in my own rgb.txt)? It seems to be an overclever
optimization (if such a word exists).

JMarc