Bug#444175: Fixed

[David Purdy]

I was mistakenly using the "add_attribute" syntax for making cells editable,
instead of "set_property".

(See section 14.4.7. "Editable Text Cells", from the python-gtk2-tutorial
documentation. Page is here on my debian system:
/usr/share/doc/python-gtk2-tutorial/html/sec-CellRenderers.html)

I changed my code over to use "set_property" instead and now I can edit cells
containing '0'.

But it may still be interesting for the devs to look into the problem with the
'add_attribute' syntax.