Make it possible to raise windows on top of the lockscreen

Requires clients to have the
X-KDE-Wayland-Interfaces=kde_lockscreenallowed_v1 set in their desktop
file, then they will be able to use the kde_lockscreenallowed_v1
protocol to raise any surface above the lockscreen.
The protocol has only 1 method, raise_surface to do exactly that.

Makes it possible to implement
teams/plasma-mobile/issues#98