Started some work on making GUI more responsive
Force Python3 version of flake8
Implemented basic progress window
Further improvements to progress reporting and cancelation support
Removed msgbox from ConverterThread, reduced it to a simple cancel() call
Added missing gui_progress_window declaration
Split GUI code into individual files
Added gui/ directory to setup scripts
Removed obsolete print()
Minor window size changes
Updated documentation a bit
Cleanup relationship between ConverterThread and GUI code