dbusServices/extensions: Take over prefs dialog from app

As outlined earlier, in order to turn the Extensions app into a properly
sandboxed application, we need to split out the extension prefs dialog
and move it elsewhere.

With "elsewhere" being the new Extensions D-Bus service, effectively
turning it into a shell extensions portal.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1106

js/dbusServices/extensions/main.js

@@ -1,9 +1,18 @@
 /* exported main */
 
+imports.gi.versions.Gdk = '3.0';
+imports.gi.versions.Gtk = '3.0';
+
+const { Gtk } = imports.gi;
+const pkg = imports.package;
+
 const { DBusService } = imports.dbusService;
 const { ExtensionsService } = imports.extensionsService;
 
 function main() {
+    Gtk.init(null);
+    pkg.initFormat();
+
     const service = new DBusService(
         'org.gnome.Shell.Extensions',
         new ExtensionsService());