status/backlight: Add new quick toggle

status/backlight: Add new quick toggle

Settings no longer exposes a slider for the keyboard brightness,
leaving keyboard shortcuts as the only way of adjusting it.

This is good enough in most cases, because devices with keyboard
backlight usually include corresponding keys on their keyboard.

However for devices without those keys, it would be good for the
settings to be exposed somewhere again. Quick settings seems like
a more appropriate place than "proper" Settings, since it gives
quick access that doesn't require a major focus change.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/6765

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2820>