Refactory the methode "MergeMailFiles"

Change-Id: I65441ade522c06335b0da23cb2fed9afc5bcb72e
Reviewed-on: https://gerrit.libreoffice.org/20714Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>
Tested-by: Katarina Behrens <Katarina.Behrens@cib.de>