feedback kcm: Use a horizontal slider like GammaRay does
Summary:
Instead of a combo box, which is a bit harder on the wording.
Also get more texts from kuserfeedback itself, no reason to translate twice.
Test Plan: Tested manually
Reviewers: Plasma, ngraham, broulik
Reviewed By: Plasma, ngraham, broulik
Subscribers: broulik, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D24470