This follows the same approach as the logout greeter, albeit a bit simplified.
It also creates one window per screen and should fix the problems we had with its positioning in multi-screen setups.

Initially I just wanted to fix blur behind the greeter on wayland and then figured I might as well split it out to reduce the likelihood of ksmserver crashing due to opengl/graphics stuff

"Switch User" in kickoff, binary launched and showed the greeter, cancelling it or clicking in empty area would quit the greeter. Switching a session also worked. The window also has blur now.

The panels show up ontop of the window on Wayland but that's also the case for the logout greeter.