Use ScreenMapper everywhere as a singleton
Summary:
There is no need to pass a ScreenMapper pointer around, as it is always the same,
it is a singleton.
Reviewers: Plasma, mwolff, hein
Reviewed By: mwolff
Subscribers: mlaurent, broulik, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D9323