The custom setter used by the slider item isn't emitting change
notifications, so the property binding that uses it as source
never propagates the new value.
Fix this by emitting proper change notifications.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2856>
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>
