When playing with the lockscreen I noticed that the emergency show does
not work on Wayland and one only gets a black screen if the system fails.

This change adds a property to the QWindow (in ksld), so that KWin can
read this and now that this is a window to be shown on the lock screen.
In addition the geometry of the background window gets adjusted correctly
on Wayland.

The change needs a small part also in KWin.