Rework compression quality settings
Summary:
Removed ticks from the slider.
Substituted the label with a SpinBox, also adding a percent sign.
Test Plan: Set compression quality using both the spinbox and the slider -> Quality of saved images changes accordingly
Reviewers: Spectacle, ngraham
Reviewed By: Spectacle, ngraham
Tags: Spectacle
Differential Revision: https://phabricator.kde.org/D24313