bnc#821208 DOC import: don't overwrite WW8Num* character styles

In general we're overwriting styles on import in case we're not pasting.
But these WW8Num* character styles are in general not from the document,
they are created because Writer needs a character style for each
numbering level to handle what's in the DOC file.

So, in case there is a character style with the same name as our
"character style for numbering" styles, prefer the later ones, as that's
intended most likely.

Change-Id: I675f867722360aca765bb96b0b43ea47deab9847
(cherry picked from commit e88ee8aa307ecec1cc22cda56b97dcfd8b838ce8)
Reviewed-on: https://gerrit.libreoffice.org/8448
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>