modalDialog: Fix fading out dialog

The dialog's state property has been read-only since
commit 2f6323afc, but the callback at the end of the
fade transition still tries to set the value directly.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/6506

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2696>

js/ui/modalDialog.js

@@ -282,7 +282,7 @@ var ModalDialog = GObject.registerClass({
             opacity: 0,
             duration: FADE_OUT_DIALOG_TIME,
             mode: Clutter.AnimationMode.EASE_OUT_QUAD,
-            onComplete: () => (this.state = State.FADED_OUT),
+            onComplete: () => this._setState(State.FADED_OUT),
         });
     }
 });