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