Qt: Create new settings dialog instance every time

closes #1

Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>
Qt: Remove unused data in downloads table

Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>
Qt: Add kB/s unit to download speed

Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>
Qt: autopep8

Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>
Better handling of downloader network/other errors