The `m_abortOperation` global variable was set by the main thread and read
by the secondary thread, i.e. undefined behavior.

QThread::requestInterruption() and isInterruptionRequest() are instead
safe because they use a QMutexLocker.

Tested only with ListJobs, are other type of jobs are currently broken
(see e.g. bugs #365869 and #365870).

Closes T3598