Backend: calibration: keep the calibration scan resolution around with the calibration

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Config: Store the scan resolution used for calibration

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Settings window: Don't always use the default calibration resolution for the calibration scan. It's not always available.
Try to use instead the closest smaller scan resolution.

Signed-off-by: Jerome Flesch <jflesch@gmail.com>