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.

Change-Id: I21bcc87a149c9f6d865ebee4012132ccc3a54af2