This makes sure that we do not try to use GstPbutils before Gst is init
and in case GstPbutils is imported while Gst is not imported, use the
`GstPbutils.pb_utils_init()` function to have the oportunity to
initialize the overrides.

Not that we also introduce a `GstPbutils.init()` variant because
`GstPbutils.pb_utils_init()` is an ugly name.