A function to set the properties of a cell instead of just using the straight mapping between the cell and the model.
This is useful for customizing the cell renderer. For example, a function might get an integer from the
tree_model, and render it
to the “text” attribute of “cell” by converting it to its written equivalent. This is set by calling
The CellRenderer that is being rendered by
The TreeModel being rendered
A TreeIter of the current row rendered