gnt: Use default 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 libgnt to use this method instead of generating its
own marshallers with glib-genmarshal.