fix(kded): center action OSD on wayland

Summary:
Currently on Wayland the Action OSD is not centered on the screen.

On Wayland we will center it on m_output (laptop, with fallback to primary and
then to first one).

X behavior is not changed.

BUG: 397586

Reviewers: romangg, davidedmundson

Reviewed By: romangg, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28916