sw: still use SvxBrushItem for page style background

Commit 6e61ecd09679a66060f932835622821d39e92f01 (Merge back branch
alg_writerframes to trunk, 2014-03-19) changed the sw UNO API, so that
in case SvxBrushItem / RES_BACKGROUND was to be get/set, it mapped that
to svx's fill style.

This has two problems: 1) it does not work (see the failing unit test)
and also page style editing UI sets SvxBrushItem, so even if the UI is
used to (actually successfully) set the page background, it won't be
saved to ODF.

So, do this background <-> fill style mapping only for frames for now.

Change-Id: I734880a40c891ae9e341cbddf8f88b0462361aac