xfce4-hdaps-dialogs.c: switch from hbox/vbox to generic box.

xfce4-hdaps-dialogs.c: switch from hbox/vbox to generic box.

The gtk_vbox_new() and gtk_hbox_new() functions have been deprecated
as of gtk+-3.2. Instead, we're supposed to use the generic gtk_box_new()
function and pass in an orientation like GTK_ORIENTATION_VERTICAL. Ok,
no problem. That change is somewhat trivial.

But, in a related change, a GtkDialog no longer exposes its content
vbox via a member variable in gtk+-3.x. So, we have to get it with the
gtk_dialog_get_content_area() accessor function. This change is also
trivial, once you have figured out that the vbox it accesses is the
same one that was removed from the public API of GtkDialog. I sure
hope it is.