[gtkmm] Gtk::TreePath interface improvement

[Daniel Elstner <daniel elstner gmx net>]

Hi,

the current Gtk::TreePath interface works, but is quite clumsy to use. 
I think this can be improved without changing the current API or ABI. 

Specfically, my intention is to make the interface similar to
std::vector<>.  Adding operator[] shouldn't be a problem, even with
reference access to the indices.  prepend_index() / append_index()
should be aliased as push_front() / push_back(), so that one can use STL
algorithms.

This is stuff is meant for inclusion in gtkmm 2.2, which will probably
be released soon after GTK+ 2.2.

Comments and suggestions are welcome,
--Daniel