editeng: harmonized types related to Tell()/Seek()

Change-Id: If0484da58011abae23ad19cbc3b9cf7e2e654b4c
Reviewed-on: https://gerrit.libreoffice.org/8216
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>
Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>