facebook: Use generic marshaller for GObject signals

Since 2.30 it's been possible to use a default c_marshaller by
passing NULL to g_signal_new(). It has since become the recommended
way of creating signals.
https://developer.gnome.org/gobject/stable/howto-signals.html

This patch ports the Facebook prpl to use this method instead
of generating its own marshallers with glib-genmarshal.