bnc#480256: OOXML import: Respect table background properties a bit more

Only getBackgroundFillProperties() (fill) was used.
Use also getBackgroundFillStyleRef() (fillRef).
Also, do not replace table background color value with cell color,
we have to interpolate the two colors (if cell color is transparent).

Unfortunately, we don't use background table property in LibreOffice, so
this seems to be a best workaround.

(cherry picked from commit 43efd9b40d40b791a2c2deedcac36b99f7efb2cf)

And add unit test.
(cherry picked from commit 5681725f1a2535a13b86104d8b8a33f750f34efc)

Change-Id: I21bcc87a149c9f6d865ebee4012132ccc3a54af2
Reviewed-on: https://gerrit.libreoffice.org/11353
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>