extensionPrefs: Request correct GDK/GTK versions

gjs no longer has an implicit dependency on GTK 3.0, so without
requesting an explicit version, we will get the highest available.

Our code isn't GTK-4 ready, so request 3.0 explicitly.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/686

js/extensionPrefs/main.js

@@ -1,4 +1,7 @@
 /* exported main */
+imports.gi.versions.Gdk = '3.0';
+imports.gi.versions.Gtk = '3.0';
+
 const Gettext = imports.gettext;
 const { Gdk, GLib, Gio, GObject, Gtk, Pango } = imports.gi;
 const Format = imports.format;