[REFACTOR] moved ISarosSessionListener and Null IMPL to core

Renamed AbstractSarosSessionListener to NullSarosSessionListener as this
class was not abstract at all.

Change-Id: Ife6c55882d5a50fa6cc8e44d0f1cdddc3453406c
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1939
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>