BonoboGenericFactory: multi/single factories

[ERDI Gergo <cactus cactus rulez org>]

I see a lot of inconsistency between multi and single factories, the
nomenclature is a mess (BonoboGenericFactoryFn vs GnomeFactoryCallback?
WTF?). While it would be clearly possible to clean this up, do we really
need single factories? They are, after all, nothing but a special case of
multi factories. You can always ignore the extra argument in your
callback.

Also, I propose adding a standard closure (a gpointer) to the callback.

Oh, and about compatbility -- I think so many things changed in G2 that
another change (that only means having to add a maximum of two extra
arguments to your callback, and is reported at compile-time) is
acceptable.

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