Parent windows for system dialogs

[Nicolas Fella]

Hi,

we have several places where we have "system" dialogs that don't have a
"real" parent window. Examples would be stuff like KWallet, network
management, or anything spawned by kded where there's no corresponding
app window. Currently, at least on Wayland, they are placed
semi-randomly on the screen, which isn't particularly nice. See e.g.
https://bugs.kde.org/show_bug.cgi?id=456060. They probably should be
centered in the/a screen.

Usually the centering would be achieved by setting  a parent window, but
in that case there isn't an obvious one. What should we do in that case?
Set the parent window to some magic Plasma window? Special-case this in
KWin somehow? Something else entirely?

Cheers

Nico