[INTERNAL] ensure thread safety

SharedEditorListenerDispatch was not thread safe regarding adding /
removing and iterating over the listeners. Just replaced the ArrayList
with an CopyOnWriteArrayList.

Change-Id: Iaacd73d06d47e7f6a611f1a2e4da91b9f40ed853
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1174
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>