jack: Uses a boxed type for the client property

This is a boxed type around the JackClient from jack itself. But as this is plugin API, application code has a hard time to actually construct a boxed value for this.

Also the way how it is registered defeats the purpose of boxed types. There is no memory management at all, this is completely unsafe would've been more correct as a G_TYPE_POINTER property:

static gpointer
gst_jack_client_copy (gpointer jclient)
{
  return jclient;
}


static void
gst_jack_client_free (gpointer jclient)
{
  return;
}

CC @wtay