bnc#821208 DOC import: fix unwanted char background in numbering char style

Word supports formatting the paragraph parker itself, and we import that
as a formatting at a position after the last character (e.g. "foo" will
have that formatting at char pos 3, which is ignored by the layout).

In addition to this hack, commit
1c22545edf9085b9f2656ca92781158b6b123db3 (Fix issue #i119405: Numbering
text style changed after importing the *.doc, 2012-08-24) added a
SwTxtNode::TryCharSetExpandToNum() hack to Writer core, where in case
such a paragraph marker attribute is set, and the SwTxtNode has a
numbering, then also modify the associated character style as well.

As that commit already noticed, there are attributes which should not be
propagated to that character style. Extend this blacklist to ignore
RES_CHRATR_BACKGROUND as well, as Word does.

Change-Id: Idcb40d37d8688c76fbd61f28428f6e3bc995f799