custom tree model, iters, and memory management

[Thomas Stover <thomas wsinnovations com>]

While working on a custom tree model (thanks to Tim-Philipp Müller for 
the tutorial), I needed more pointers in my iter structure than the 
user_data, user_data2, and user_data3. So I defined a new structure and 
stored that in user_data. For the model's _get_iter(), _iter_children(), 
_iter_parent(), iter_nth_child() methods, if the "output" iter's 
user_data is NULL, a new one of these "extra pointers structures" gets 
allocated. At the moment, all of these allocations get also referenced 
in a GSList in the custom model, and freed when the model is finalized. 
While this does work ok, it feels strange. Any thoughts? Some light 
analysis shows the treeview doesn't use that many iters, so maybe this 
is fine?