When inserting a new document into current position we need to
concat to lists into one.

Added unit test.

Change-Id: I10689256e0ffc5cf93722b1d45f09f610211b14a
Reviewed-on: https://gerrit.libreoffice.org/46978Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>