GraphicManager: let's assert GraphicObjects are inserted and deleted once

Change-Id: Ie3bfb78434fa25fdbbdfb90cb9590649fec2dc28
Reviewed-on: https://gerrit.libreoffice.org/11214
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>