[INTERNAL] moved observables to core

Also refactored and renamed the InvitationProcessObservable class.

TODO: Those global get/set classes *feels wrong* !

Change-Id: I185b4011edea84f27e60ccca0056f50aa1f853c5
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1651
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>