Change UI for setting jpeg quality to slider

Summary:
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.

Test Plan:
Set high quality in settings, don't change in save dialog -> Output image has high quality
Set high quality in settings, override in save dialog with low quality -> Output image has low quality
Set low quality in settings, don't change in save dialog -> Output image has low quality
Set low quality in settings, override in save dialog with high quality -> Output image has high quality

Reviewers: ngraham, VDG, Gwenview

Reviewed By: ngraham, VDG, Gwenview

Subscribers: Spectacle

Tags: Gwenview

Differential Revision: https://phabricator.kde.org/D24309