Draft: Extract Wayland-specific bits of KWindowSystem into KWaylandExtras

Open Nicolas Fella requested to merge work/nico/wayland-extras into master

KWindowSystem has some Wayland-specific code for using XDG activation that can't easily be abstracted to cover X11 too

Extract that into a KWaylandExtras class, similar to how we extracted X11-specific code into KX11Extras

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