[gtkmm] gtkmm API change: Adding TreeView::move_column_to_start()

[Murray Cumming <murrayc murrayc com>]

I'm giving myself permission to break the API-freeze to add
TreeView::move_column_to_start(), which is the equivalent of 
gtk_tree_view_move_column_after(treeview, column, 0).

We need to wrap these null-value cases separately because our normal
methods don't allow null values (we use C++ references). But sometimes
we miss one.

-- 
Murray Cumming
www.murrayc.com
murrayc murrayc com