Sets the default comparison function used when sorting to be sort_func.
If the current sort column id of sortable is
GTK_TREE_SORTABLE_DEFAULT_SORT_COLUMN_ID, then the model will sort using
this function.
If sort_func is NULL, then there will be no default comparison function.
This means that 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 sortable
is GTK_TREE_SORTABLE_DEFAULT_SORT_COLUMN_ID, the model will be unsorted.
Sets the default comparison function used when sorting to be sort_func. If the current sort column id of sortable is GTK_TREE_SORTABLE_DEFAULT_SORT_COLUMN_ID, then the model will sort using this function. If sort_func is NULL, then there will be no default comparison function. This means that 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 sortable is GTK_TREE_SORTABLE_DEFAULT_SORT_COLUMN_ID, the model will be unsorted.