A GtkTreeModel wrapper that sorts the underlying model.
The GtkTreeModelSort is a model which sorts a given submodel.
It does not hold any data itself, but rather is created with a child
model and proxies its data. It has identical column types to this
child model, and the changes in the child are propagated. The primary
purpose of this model is to provide a way to sort a different model
without modifying it.
Creates a new sortable tree model based on another one.