Sets the default comparison function used when sorting to be f. If the
current sort column id of this sortable is
GTK2.TREE_SORTABLE_DEFAULT_SORT_COLUMN_ID, then the model will sort using
this function.
if f is 0, then there will be no default comparison function. This means
once the model has been sorted, it can't go back to the default state. In
this case, when the current sort column id of this sortable is
GTK2.TREE_SORTABLE_DEFAULT_SORT_COLUMN_ID, the model will be unsorted.