Improve appearance of the logout dialog on wayland
Summary:
The logout dialog has a taskbar entry and appears like a regular window on
wayland. This is because Qt does not support fullscreen windows with xdg_shell
as of 5.9.4. wl-shell works, so force it for the time being.
Test Plan: Ran ksmserver-logout-greeter on X11 and wayland, looks identical now.
Reviewers: Plasma, graesslin, davidedmundson, bshah
Reviewed By: Plasma, graesslin, bshah
Subscribers: ngraham, bshah, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D10425