GtkTreeModel for cl-gtk2

I've added to cl-gtk2 a Lisp implementation of the GtkTreeModel interface for trees. This enables the use of the GtkTreeView widget to display hierarchical table-structured data. I don't consider supporting Gtk's implementation of GtkTreeModel to be necessary since cl-gtk2 now has its own implementation so I've decided to skip it.

This screenshot depicts a visualization of a parse tree for Lisp expression. It is included in cl-gtk2 as a demo and can be invoked with:

(gtk-demo:demo-treeview-tree)

I've settled on GNU TexInfo as a way of making documentation. Amongst all the documentation tools that make documentation from source code and documentation strings, I couldn't find anything suitable so I decided to stick with something simpler. I borrowed a documentation template from CFFI and, based on this template, I've documented the GObject binding.