On showing windows where the users want them, on wayland

Dear KWin'ers,
There's something I've been missing lately, which is the ability to
have krunner/yakuake show on the screen the cursor is.

At the moment on wayland we're relying on QCursor::pos() and
qGuiApp->primaryScreen(), which are both busted, so we need an
alternative.

I was thinking that maybe we can:
- make Qt report the screen where the current cursor is the primary
one, although I believe we don't have such a concept part of the
standard protocols, so we'll have to include it somehow making it
rather intrusive for our limited use case.

- include in the KWayland::Client::PlasmaShell a way to report the
screen (e.g. the edid or another identifier) with the current cursor.

I'm leaning towards the latter, could mean versioning the protocol but
since we control both sides it shouldn't be that hard.

Happy easter, wish you tons of chocolate. 🐰
Aleix