Summary:
The internal QWindowsPipeWriter in Qt doesn't do any buffering, so the first 8
bytes (the tag) were sent first, and the rest only later.

This solution uses an explicit flush() method, while doing the flushing
in the destructor would have been much more convenient and less
error-prone, but the flushing can throw an exception, so we need
to do it inside the try/catch -- and certainly not when the stream
is deleted because another exception happened. This would all be
so much simpler without the use of exceptions :-)

Test Plan: all tests pass (on Linux)

Reviewers: dvratil, kfunk

Reviewed By: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D29265