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