Add support for new Notification role on PlasmaSurface

Summary:
Sets notifications on all desktop and doesn't activate them.

Unfortunately doesn't work on Plasma yet. Seems the windows don't get
tagged properly.

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D1930