D23106 added a spinner box to set a save quality for JPEG images. I changed it to a slider to be consistent with the same functionality in Spectacle, but I didn't use the ticks because it makes the slider me misaligned with the respective label. Also, I used % as the label unit. I think it is more clear and will therefore propose the same for Spectacle.
I did not really understand how in the original commit the value from the spinner box was saved to the config, so I set it explicitly when the slider value changes.