Add output and connected outputs config API

Summary:
In order to support writing and retrieving config data by API consumers
introduce two API exposed functions for calculating hash values of
individual outputs and configs dependent on the currently connected
outputs.

This allows to move the similar code path out of kscreen.

Test Plan: Compiles.

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D16843