status/backlight: Notify slider value changes

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>

js/ui/status/backlight.js

@@ -44,9 +44,14 @@ const SliderItem = GObject.registerClass({
     }
 
     set value(value) {
+        if (this.value === value)
+            return;
+
         this._slider.block_signal_handler(this._sliderChangedId);
         this._slider.value = value / 100;
         this._slider.unblock_signal_handler(this._sliderChangedId);
+
+        this.notify('value');
     }
 });