... use the xml token "use-window-font-color" instead of "color"
to encode a boolean value of whether an auto color is used or not.
This is because the "auto" color is 0xffffffff and cannot be
written into "color" xml field which can store only 3 byte-hexcode.

This commit also adds a unit test in subsequent_export-test.cxx
to ensure the correct export of automatic color.

Change-Id: I42aab926f31669c1423bc09b96b45c410c9732de
Reviewed-on: https://gerrit.libreoffice.org/41252Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Dennis Francis <dennis.francis@collabora.co.uk>