non-obvious GtkTreeView API change

[Jonathan Blandford <jrb redhat com>]

Just so people know, I changed GtkTreeView and GtkTreeViewColumn to
ref/sink cells and columns.  This means that code that formerly did:

renderer = gtk_cell_renderer_text_new ();
column = gtk_tree_view_column_new_with_attributes ("title", renderer, NULL);
gtk_tree_view_append_column (tree_view, column);
g_object_unref (renderer);
g_object_unref (column);

Can now lose the two unref steps.

Thanks,
-Jonathan