This makes sure that the data is completly written
to the Process before waiting for the process
to finish. If this is not done the output will be written
in small chunks trough the eventloop causing an uncessary
delay before the process on which we wait has even a chance
to work.
This delay is mainly noticabe on Windows where it can
take ~30 seconds to write out a 10MB file in the 512 byte chunks
that are provided by gpgme in QProcess::write calls.
Also fixes a problem where sometimes the last chunk was not
sent before the Output process closed.