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