opensubtitles: Fix downloading of subtitles

In 15872af34c156f68a6da484ca8ce139d44fa8f8a we started using
GLib instead of PyXDG, but didn't import GLib.
opensubtitles: Fix error with newer version of PyGObject

We cannot pass an object as user_data, as pygobject stopped handling
non-integers, non-capsules or not None as possible arguments.

As we do not use Totem.plugin_load_interface()'s signal connection
feature, pass None instead.

https://bugzilla.gnome.org/show_bug.cgi?id=724093