Support fractional scailng in KScreen Scaling preview

Summary:
Qt 5.something changed to support fractional scaling.
I updated the backend code to support that, I never updated the preview
UI.

Test Plan: Moved the slider, and it looked awesome

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5412