xfce4-hdaps-dialogs.c: prefer xdg-open to exo-open.
There are two places we handle URLs in the plugin, and they were using
different programs to open a URL. This commit makes them both use
"xdg-open" instead of "exo-open". I have no idea if it lives up to the
promise, but xdg-open is supposed to intelligently determine your
desktop environment and then do The Right Thing; whereas exo-open
is specific to XFCE.