Traditional user interfaces can rely on this interface to define the foundations of typical desktops. Currently it's possible to set up background, panels and locking surfaces. Tell the shell we want it to create and set the lock surface, which is a GUI asking the user to unlock the screen. The lock surface is announced with 'set_lock_surface'. Whether or not the shell actually implements locking, it MUST send 'unlock' request to let the normal desktop resume. Only one client can bind this interface at a time. A screensaver surface is normally hidden, and only visible after an idle timeout.