Use G_ADD_PRIVATE et al. when available.

Use G_ADD_PRIVATE et al. when available.

As of GLib 2.58, using g_type_class_add_private has been deprecated;
it seems that type implementations are expected to use the G_ADD_PRIVATE
or G_ADD_PRIVATE_DYNAMIC macros.

Now that we use G_DEFINE_TYPE_xxx, it is reasonably straightforward
to do this.  Fall back to the old method for older GLib where these
macros are not available.