WIP locker: Rely on cosmic-bg for showing background

[ids1024]

Requires pop-os/cosmic-comp#2792 and pop-os/cosmic-bg#144.

Currently this shows the cosmic-bg background with a frosted glass effect, which is interesting but probably not what we want. It may also be best to no longer user a subsurface for the UI, now that the main surface isn't used for the background.

If cosmic-bg isn't running or the compositor doesn't support the protocol, the background will simply show the clear color the compositor uses for lock surfaces.

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