Both BacklightBrightness and DDCutilDisplay now derive from this. ScreenBrightnessController now updates a list of DisplayBrightness objects made possible by a new DDCutilBrightness::displaysChanged signal.
Steps towards #19.
Note to reviewers: !325 and !327 contain the first parts of this branch, review these first if the full branch feels too much. This MR will get a little simpler once they're merged.
In the original revision, the interface was called PowerDevil::ScreenBrightness. I took out the PowerDevil namespace because it looks awkward when no other related class is using it, and might go away anyway if we factor out some brightness parts into a common library usable also by KWin. I renamed ScreenBrightness to DisplayBrightness because we're already dealing with DDCutilDisplay as a Display, and the list is called displays(), and frankly it's just a better name despite ScreenBrightnessController existing already.