Draft: daemon: Introduce per-display ScreenBrightnessControl D-Bus API
Depends on !359 (merged) and prior MRs in the chain. Exposes the relevant new API to fix #19 and allow the Brightness applet to show brightness controls for each display.
It builds. It restarts. It makes sense to me. Does it work? Who knows! Let me adapt the Brightness applet to use this new API, and we'll see if it breaks completely.
I started out with a proper split of global + display-specific interfaces, and regular D-Bus properties instead of D-Bus methods. The display-specific D-Bus XML spec is still in the patch for now, but I switched to the simpler method-with-display-id pattern to accelerate implementing a first working version. Now I'm thinking this might also be easier to use for the applet, our main API user. I haven't designed D-Bus APIs before, not fully sure about the trade-offs, open to feedback.