Writing a CellRenderer

[ggirard about org]

Hello,

I have a simple question and it should have been answered before, but I can't find any acceptable answer.

How can I write my own CellRenderer ?

I would prefer to do it in Ruby or something like that but if I have to use C it's still ok.

For example, I want to be able to edit cells without having to switch to the edit mode. My tree would be 
permanently editable as a whole, as in a text editor.

Any idea ?

Best regards