Introduce generic Output class
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.
Differential Revision: https://phabricator.kde.org/D11781