Accessor of the model of the gtk-tree-view class.
The gtk-tree-view-model slot access function returns the model the tree view is based on. Returns nil if the model is unset.
The (setf gtk-tree-view-model) slot access function sets the model for a tree view. If the tree view already has a model set, it will remove it before setting the new model. If model is nil, then it will unset the old model.