Position output identifiers with respect to scaling factors

Summary:
Respect scaling factors for calculation of screen sizes and by that fix
off-center placement of output identifiers.

Test Plan: Works on X, on Wayland the positioning does not work in any case currently.

Reviewers: davidedmundson, broulik

Reviewed By: davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D12151