[REFACTOR] cleaned up classes in subscription package

Removed not needed IncomingSubscriptionEvent class.

Changed add/remove*Listener method names.

Change-Id: I2996fc26de189c0fc25616b7513b9f81f8b8b580
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1307
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>