Introduce output replication API
Summary:
The API allows an output to be replica of another output by setting a
replication source on the former.
Supporting backends are supposed to handle this by moving and resizing the
replica to match the source geometry.
Test Plan: Compiles.
Reviewers: KWin
Subscribers: davidedmundson, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D23710