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 tree_view, 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
tree_view reverts to the default behavior of allowing all columns to be
dropped everywhere.
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 tree_view, 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 tree_view reverts to the default behavior of allowing all columns to be dropped everywhere.