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 GtkTreeViewColumns 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 GtkTreeViewColumns 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.