This gets us closer to supporting brightness controls for multiplemonitors individually. DDCutilDisplay gets a very similar API asthe recently introduced BacklightBrightness (which represents asingle backlit display). ScreenBrightnessController still hasseparate codepaths for both, but they look essentially the same now.
In the future, this will allow us to derive both DDCutilDisplay andBacklightBrightness from the same display base class and makeScreenBrightnessController work with a list of such objects. (Note: If you're interested in those follow-ups, I'm putting them on a branch until this is merged.)
Untested so far, because I'm currently away from my regular monitor for a few weeks. Hopefully I can get access to one that supports DDC so I can test it there. Should be fine because it's just a refactoring of existing code, but who knows what bugs I'm sneakily introducing.