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