Propagator: make sure every network job has a parent

Propagator: make sure every network job has a parent

This could make sure that the network job gets deleted if the parent job gets
deleted, and would avoid crashes like:

Crash: EXCEPTION_ACCESS_VIOLATION_READ at 0xffffffff8b008a04
  File "qiodevice.cpp", line 1617, in QIODevice::errorString
  File "propagatedownload.cpp", line 264, in OCC::GETFileJob::slotReadyRead
  File "moc_propagatedownload.cpp", line 85, in OCC::GETFileJob::qt_static_metacall
  File "qobject.cpp", line 3716, in QMetaObject::activate
  File "moc_qiodevice.cpp", line 154, in QIODevice::readyRead
  File "qnetworkreplyhttpimpl.cpp", line 1045, in QNetworkReplyHttpImplPrivate::replyDownloadData

(#5329)