When built on Ubuntu 16.04, all editions of DupeGuru open an empty progress dialog few seconds after application is started.
qt: instantiate QProgressDialog on-the-fly rather than on startup
This fixes a glitch where the progress dialog would needlessly show up
on startup. It's also, incidentally, a better practice.
This updates `progress_window`, which fixes a bug where the progress
window would be mistakenly shown on starup.