bug#9722: list-colors-duplicates does not exclude enough colors on Windows

[Eli Zaretskii]

> From: Juri Linkov <juri@jurta.org>
> Cc: Eli Zaretskii <eliz@gnu.org>,  9722@debbugs.gnu.org
> Date: Wed, 19 Oct 2011 11:20:14 +0300
> 
> Or we could add a new system-independent variable to hold system colors
> (and add GTK colors to it too).

GTK is available on Windows as well, so someone someday could port
the GTK Emacs to Windows.  IOW, if we just add the GTK colors, they
should be visible on all platforms.