Introduce generic Output class

Summary:
In order to separate high-level properties of individual outputs from
hardware-specific ones and access these, introduce a new generic class Output.

Also make the DrmOutput class directly a child class of this generic class.

The long-term goal is to get rid of the Screens global object on Wayland and
instead directly work with Output objects on compositing level.

This should enable us long-term to do direct scanout to hardware planes, what
I predict needs this generic output representation at one point.

Test Plan: Manually.

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D11781