[FIX] files modified by document provider are no longer transmitted


The ProjectDeltaVisitor only checked if the file is currently
opened by an editor. This may not always be the case.

Change-Id: I6f2bbfb972e26b8fbafda30044e4245b378ea38e
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/637
Tested-by: Jenkins-Eclipse QA
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>