According to issue #608 we were silently overwriting files when there
was a suffix being generated for a chosen export filename when one was
missing to begin with. Unfortunately, this helpful feature avoids all
the conveniences offered by QFileDialog in regards to alerting the user
to overwriting an existing file. Worse still, this feature already
exists in QFileDialog and the native APIs it can rely on.
This patch reimplements QFileDialog.getSaveFileName to allow the use of
the default suffix feature as functions.getSaveFileNameWithSuffix and
removes most of the magic involved with the old solution.
