This patch was originally written by
Zach Goldberg <zach@zachgoldberg.com> with modifications and
review by Simon van der Linden <svdlinden@src.gnome.org> and
Colin Walters <walters@verbum.org>.

This impementation enforces the assumption that any one function
signature can only have one (callback, userdata, destronotify) tuple.
This allows us to move callback creation into the actual function
invoke pipeline and also to keep just one destroy notify callback
around, vastly simplifying the code.

https://bugzilla.gnome.org/show_bug.cgi?id=603095