This implements a backlight manager which creates a global and multiple
per monitor BrightnessScales and updates the backlight based on the
BrightnessScales changing and the backlight changing. It shows the OSD
in response to changes.
In the future it will also handle dimming, auto brightness and keyboard
shortcuts.
The idea is that the per monitor scale changes the ratio between monitor
brighnesses and the global scale scales the monitor brightnesses
accordingly.