Associate output transforms and orientations

Summary:
We use internally Qt:ScreenOrientation for representing output transforms.

This is not ideal since the values do not map directly to Wayland transform
values, but we can make it work by using OR combinations of
Qt:ScreenOrientations.

Do this for now and see if we should not better introduce an internal enum
mapped directly.

Additionally the OR combinations need to be handled in the drm backend at
various places accordingly as well (see TODOs).

Test Plan: Compiles

Reviewers: KWin

Subscribers: kwin

Tags: KWin

Maniphest Tasks: T11670

Differential Revision: https://phabricator.kde.org/D25505