This implements the zwlr_output_power_management_v1 protocol, which
allow clients such as swayidle to toggle display power state. Outputs
are intentionally not removed per the discussion in #245.
A dpms_powered_off flag is tracked on each output to prevent a DPMS "on"
request from re-enabling an output that was disabled via wlr-output-management
rather than DPMS.
The protocol generation/dep additions is heavily inspired by swaywm
Fixes#245
