SwXTextFrame: restore semi-lost BackColor property

In the past in case FillStyle was set, BackColor wasn't ignored, now it
is, since 6e61ecd09679a66060f932835622821d39e92f01 (Merge back branch
alg_writerframes to trunk, 2014-03-19).

The intention there was that in case FillStyle is set, FillColor will be
used: so to keep both new users and old filters happy, just restore
handling of BackColor in case FillStyle is set, but not FillColor.

Change-Id: Ie034a959e72e48b617cf598af8f7afd030bb50c4