brightnessManager: Handle dimming and auto brightness

brightnessManager: Handle dimming and auto brightness

We don't want gnome-settings-daemon to poke directly at mutters
backlight anymore because that would be a split brain situation where
g-s and g-s-d will fight with each other. This implements a way to tell
the BrightnessManager to dim the screen and set an auto brightness
target, and the manager will set the backlight accordingly.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3751>