Use out-of-band communication between ksld and greeter

The screenlocker_greet needs to tell the parent ksld process which
windows it created. Ksld sends input events to these windows. So
far this was based on an X property on the window. Unfortunately
ksld didn't validate whether the windows tagged with this property
belong to the screenlocker_greet process it started.

With this change the communication for announcing windows is moved
away from the X11 protocol and instead a custom Wayland protocol is
used.

Ksld starts a KWaylandServer when the greet process gets started. It
creates anonymous unix sockets for the connection and passes one
filedescriptor to the started greeter process.

The check for the X property is removed in ksld and instead only
windows ids passed through the Wayland socket connection are
accepted.

REVIEW: 121429