[INTERNAL] moved misplaced EditorAPI logic

Moved logic to handle part listeners into the EditorManager.

Change-Id: Iccf784b69da1963238fa2e116bc0a7a8b0754643
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1899
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>