extensionPrefs: Schedule updates check on activate

While gnome-shell will now check for extension updates, the checks
are performed infrequently. Opening the Extensions app implies that
the user's current focus is on extensions, so it is an appropriate
time to schedule another updates check.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/1968

js/extensionPrefs/main.js

@@ -38,6 +38,7 @@ class Application extends Gtk.Application {
     }
 
     vfunc_activate() {
+        this._shellProxy.CheckForUpdatesRemote();
         this._window.present();
     }