wip: DPMS private protocol

This is private protocol to test out monitor blank/unblank (will be turned into a zwlr_* thing)

I wonder if dpms is still a think or if we should simply go for blank/unblank. The reason for not using zwlr-output-management is that the later also reconfigures the compositor space (as it should) but we don't want that on power saving - also we ought to act on known outputs and not on disabled ones.

Ideally we'd not merge this but use a wlr-* protocol right away: https://github.com/swaywm/wlr-protocols/issues/56