Reminder that this is the next MR needing review / not yet approved in my per-display brightness series.
Rebased without conflicts, and included an extra commit to also expose the "legacy" setBrightness() set of displays. This second commit was originally included in !360, but it fits better here, thematically speaking, so I moved it forward. I'll remove it from the later MR.
We can squash both commits into one (they're that close) or leave them separate.