Emit signals when a screen is added or removed
Summary: This is the implementation of the new Plasma Corona API, as described in https://phabricator.kde.org/D8566
Reviewers: Plasma, ervin, dvratil, mlaurent, hein, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: mwolff, apol, mart, davidedmundson, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D8567