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