Fix #895 by adapting to Qt5 and cleaning up properly

Relevant Qt commit:
https://codereview.qt-project.org/#/c/72637/
"QProgressDialog: don't require setValue(0) to be called."

"Fixed by starting the timer in the constructor (most code doesn't reuse
progress dialogs, so this fixes the most common case)"

This messes us up because we're (ab-)using the dialog
in a non-standard way.

https://bugreports.qt.io/browse/QTBUG-47042
"QProgressDialog is designed to show itself automatically, based
on an internal estimate for the duration of the operation and the
minimumDuration property. You never call show() or exec() on it
manually. You're also not supposed to keep it around when it's not
used. In 5.4, the only way to start the internal duration estimation
was to call setValue(0). But we noticed that many people didn't call
setValue(0)"

Workaround here:
https://bugreports.qt.io/browse/QTBUG-47042

"As a workaround I added the line m_progressDlg->reset(); in the main
window constructor in order to get the same behavior as in former
versions."