xfce4-hdaps-dialogs.c: replace email/url hooks with a signal handler.

The old gtk_about_dialog_set_{email,url}_hook() functions that we used
for our custom http(s):// and mailto:// handlers are gone. To achieve
the same goal, they've been replaced by an "activate-link" signal
handler, and both implementation (that is, the hooks) have been
combined into a single call to xdg-open.