Add cosmic-session-lock-layer-v1 protocol

[ids1024]

pop-os/cosmic-comp#2792

[Drakulix]

I assume we add notify events in a version 2 later.

[ids1024]

My thought is to have a separate cosmic-session-lock-notify-v1 protocol and global for that, since notification that the session is locked is not privileged, while creating session lock layers (or layer surfaces in general) is. For instance we want the panel to expose the notify protocol to applets.

(Technically it would also work to just expose them in one protocol, but the layer part is inert for clients without wlr-layer-shell-unstable-v1; but it's cleaner to have them separate.)