"threaded" balaced GTree in Glib

[Maurizio Monge <maurizio monge gmail com>]

hello,
i have converted GTree to a threaded balanced tree (see
http://www.stanford.edu/~blp/avl/libavl.html/Threads.html),
and this means that it is slightly slower (in particular the 
function to remove a node is a bit involuted), but it is much 
easer to remove and add nodes while traversing the tree
without requiring a bigger node structure and without having
to use callbacks, thus making the library much more 
comfortable to use.
It looks like glib does not get much attention on this ml (i
have sent a couple of patches for something i considered
useful and got no answer, if you think my code is broken
just tell me so :-) ).
If you are interested just tell me.
I trust your wisdom.

Maurizio Monge