set dpi_600 to True by default

[larsfroelich]

The dpi_600 setting allows improved print quality at the expense of speed. However, it's still very fast and can significantly improve contrast on the prints, so I argue it should be enabled by default. This could also be a checkbox somewhere in the UI later down the line.

[larsfroelich]

see also this line in brother_ql https://github.com/pklaus/brother_ql/blob/56cf4394ad750346c6b664821ccd7489ec140dae/brother_ql/conversion.py#L60

[FriedrichFroebel]

AFAIK, this breaks die cut labels due to mismatching dimensions (https://github.com/pklaus/brother_ql/blob/56cf4394ad750346c6b664821ccd7489ec140dae/brother_ql/conversion.py#L102 https://github.com/pklaus/brother_ql/blob/56cf4394ad750346c6b664821ccd7489ec140dae/brother_ql/conversion.py#L125-L126), while for endless labels, this seems to downscale the image to 150 dpi and then upscale it to 300 dpi again: https://github.com/pklaus/brother_ql/blob/56cf4394ad750346c6b664821ccd7489ec140dae/brother_ql/conversion.py#L109-L113