[NTG-context] colo-ema defines X Window colors

Hello Hans, I noticed that the color def file colo-xwi is part of colo-ema. After a look in the source of colo-ema it is now clear, that colo-ema defines the X window colors. I wrote this color defs a few years ago for latex and converted them to context without looking in the origin file again. Sorry for that... So I suggest to rename colo-ema in colo-xwi (but it's your decision). I append a slightly changed version. Regards, Peter