portalHelper: Require GTK 3.0

When multiple versions of a typelib are available, GI will pick the
highest one unless an explicit version is requested. So as GTK 4.0
is becoming a thing, it's time to do that.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/970

js/portalHelper/main.js

@@ -1,4 +1,6 @@
 /* exported main */
+imports.gi.versions.Gtk = '3.0';
+
 const Format = imports.format;
 const Gettext = imports.gettext;
 const { Gio, GLib, GObject, Gtk, Pango, Soup, WebKit2: WebKit } = imports.gi;