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