Add more private function exclusions to config.jnaerator

There is likely a better way to do this, namely recognising the
G_GNUC_INTERNAL attribute. Skip them explicitly until it is figured
out how to do this.

The BridJ warnings about functions the addresses of which are not
found we prevent by doing this are those that we get when trying
to use the GObject bindings.
Manually define GData as void

This is a workaround to JNAerator not correctly handling (due to
misconfiguration or a bug) opaque structs, at least in the form

typedef _Name Name;

With this workaround, GData* is handled as void*. This fixes the
BridJ.sizeOf() of classes that use it, at the expense of losing
type information.
Add Singleton test

Ported from GLib's own singleton.c.
Add GPL and LGPL licenses