On showing windows where the users want them, on wayland

[David Edmundson]

The relevant extension is now moving into official wayland-protocols.

Given we know we do want this eventually we may as well implement it
now on the kwin side so we're ready.
We can then use it for a few OSDs and maybe krunner/yakuake as those
are the places where plasmashell doesn't currently work as we want
this active screen feature.

Even if the Plasma panels are best left till Plasma 6, we can get a headstart.

I'll open some phabricator tasks.

David