xfce4-hdaps-dialogs.c: consolidate the URI launcher.

Now that the signal handler for the "about" dialog works, let's
consolidate the two identical URI launcher routines. The new
factored-out launch_uri() function tries all three approaches
in succession:

  1. whatever gtk_show_uri_on_window() does,
  2. exo-open, and
  3. xdg-open.

If those all fail, it fails. We had to twiddle some things to allow us
to pass a GtkWindow pointer into the launch_uri() function because
gtk_show_uri_on_window() needs one, but that wasn't a big deal.