Port QPA away from Wayland

Summary:
So far wayland was used by internal clients to submit raster buffers
and position themselves on the screen. While we didn't have issues with
submitting raster buffers, there were some problems with positioning
task switchers. Mostly, because we had effectively two paths that may
alter geometry.

A better approach to deal with internal clients is to let our QPA use
kwin core api directly. This way we can eliminate unnecessary roundtrips
as well make geometry handling much easier and comprehensible.

The last missing piece is shadows. Both Plasma::Dialog and Breeze widget
style use platform-specific APIs to set and unset shadows. We need to
add shadows API to KWindowSystem. Even though some internal clients lack
drop-shadows at the moment, I don't consider it to be a blocker. We can
add shadows back later on.

CCBUG: 386304

Reviewers: #kwin, davidedmundson, romangg

Reviewed By: #kwin, romangg

Subscribers: romangg, kwin

Tags: #kwin

Maniphest Tasks: T9600

Differential Revision: https://phabricator.kde.org/D22810