fdo#64956: editeng: fix RTF color table export

The editengine RTF export produces this:
{\colortbl\red255\green255\blue255;;}

... and then it proceeds to map COL_AUTO to \cf0 i.e. the "white" entry
that is the result of erroneously writing out the 0th entry regardless
of whether it is COL_AUTO or not.

Fix the color table export to always put COL_AUTO first (as the Writer
RTF export already does), and simplify the code a bit.

(cherry picked from commit 55070972b32e719e4855855797263d6342a3625f)

Conflicts:
	editeng/source/editeng/editdoc.cxx
	editeng/source/editeng/editdoc.hxx
	editeng/source/editeng/impedit4.cxx

Change-Id: Ia8ce19f387e3627a1b4a26bcc723edcf5b1ffdf8
Reviewed-on: https://gerrit.libreoffice.org/9585
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>