Regardless of the value of the View -> Field Shadings option, shadings
were always lost when saving to HTML.

Implement handling of this in the HTML conditionally, so in case that
UI option is on, then shadings are preserved in the HTML result;
disabling that option results in the old behavior, though.

Change-Id: I1bd19f4c6e22aff2f84fac25f0a506ad0127cc3c
Reviewed-on: https://gerrit.libreoffice.org/69081Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins