xfce4-hdaps-dialogs.c: construct "about" dialog by hand.

The way we were constructing our "about" dialog made it impossible to
hook up the "activate-link" signal handler to it. It pretended to
work, but attempting to add the signal handler to an XfcePanelPlugin
silently failed, and the only way to get a real GTK object (to make it
work) is to build up the "about" dialog ourselves.