dbusServices/extensions: Save stateObj on extension

We don't need it for anything for the time being, but reducing the
differences between extension object in the gnome-shell process and
in preferences still seems a good idea.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2832>

js/dbusServices/extensions/extensionPrefsDialog.js

@@ -36,6 +36,8 @@ export const ExtensionPrefsDialog = GObject.registerClass({
         const prefsModule = await import(prefsJs.get_uri());
 
         const prefsObj = new prefsModule.default(metadata);
+        this._extension.stateObj = prefsObj;
+
         prefsObj.fillPreferencesWindow(this);
 
         if (!this.visible_page)