[INTERNAL|API] added IEditorManager interface

Changed OPN to use the new interface.

Change-Id: I02f94189476ba85f09b72aa468bbee979b482a62
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1936
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>