Change UI for setting jpeg quality to slider

Summary:
[[ https://phabricator.kde.org/D23106 | 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 [[ https://phabricator.kde.org/D23106 | 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