status/backlight: Fix keynav within discrete item

When representing the keyboard backlight levels as discrete items,
it is currently not possible to keynav past the first button.

Fix this by bypassing the usual event bubbling and passing key press
events directly to the focus manager, similar to what we do for
popup menu items.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3545>

js/ui/status/backlight.js

@@ -133,6 +133,10 @@ const DiscreteItem = GObject.registerClass({
         this._levelButtons.set(key, box);
     }
 
+    vfunc_key_press_event(event) {
+        return global.focus_manager.navigate_from_event(event);
+    }
+
     _syncLevels() {
         this._levelButtons.get('off').visible = this.nLevels > 0;
         this._levelButtons.get('high').visible = this.nLevels > 1;