Qt: remove useless printing in D/L workers
Qt: download_manager.py: clean Python
Qt: download_manager.py: reorder classes
Qt: implement utils for UI loading
Qt: UI loading: use a class instead
Qt: utils: add `QCommonDialog`
Qt: choose_bitrate_dialog.py: clean Python
Qt: utils.py: remove unused function
Qt: app.py: clean Python
Qt: "config" -> "settings"
Qt: add config file
Qt: clean Python here and there
Clean BOs
Some PEP-8 and Python cleaning