Use Transform enum internally

Summary:
Instead of using Qt::ScreenOrientation use an enum class that is directly
mapped to KWayland's transformation enums. This simplifies the code.

Test Plan: Compiles and transformations work as before.

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: zzag, kwin

Tags: #kwin

Maniphest Tasks: T11670

Differential Revision: https://phabricator.kde.org/D25558