daemon: Add per-display access methods to ScreenBrightnessController

This isn't quite a final D-Bus API yet, but it goes a long way towards implementing one in a later commit. See #19 (closed) for more on this topic.

Main changes:

• id() and label() methods added to DisplayBrightness interface.
• ScreenBrightnessLogic is now tracked per display.
• Display changes are emitted from onDetectorDisplaysChanged().
• ScreenBrightnessController methods now expect a displayId.
• Legacy API without displayId operates on displays of the first non-empty detector, and calls non-legacy API with displayId(s).