[REFACTOR] move Session/ProjectNegotiation base classes to core

Change-Id: Ie4c5fbbf4c31aec5916f2a0f50b7b54e4a7e983c
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1633
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>