Sets a user function for determining where a column may be dropped when dragged.
This function is called on every column pair in turn at the beginning of a column drag to determine where a drop can take place. The
arguments passed to func are: the this, the `GtkTreeViewColumn` being dragged, the two
`GtkTreeViewColumn`s determining the drop spot, and user_data. If either of the `GtkTreeViewColumn` arguments for the drop
spot are null, then they indicate an edge. If func is set to be
null, then this reverts to the default behavior of allowing all columns to be dropped
everywhere.
| this |
A `GtkTreeView`. |
| func |
A function to determine which columns are reorderable |
| destroy |
Destroy notifier for |
| user_data |
User data to be passed to |