Until now, the getPart() in writer was mis-used to return the current page
number.  This breaks assumptions about parts on many places, like that
that invalidates in one part should not affect invalidates in the other part,
and similar.

We have been special casing that on many places for Writer, but we should fix
the API instead (finally) - should we need the current page number, we should
introduce a new call for that, the current situation is just confusing.

For now I kept the rest of the part-related calls untouched in Writer, as
they are less harmful.

Change-Id: I1c996cbc8a3c33778e68e5f9c20f62d57289b5c7
Reviewed-on: https://gerrit.libreoffice.org/36259Reviewed-by: Jan Holesovsky <kendy@collabora.com>
Tested-by: Jenkins <ci@libreoffice.org>