Remove some uses on QDesktopWidget

Summary: Adds an unnecessary dependency to QtWidgets and forces some round-trips to the X server that we don't need.

Test Plan: Been using it for some days

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: hein, davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22738