Editable SpinButton on a TreeView: detect *every* changes

[Pozz Pozz <pozzugno gmail com>]

I created with success a TreeView based on a ListStore. One column has a
SpinButton (actually a GtkCellRendererSpin).

Now I want to detect when the user *is* changing the value, i.e. every time
he clicks on + or - buttons.

If I connect the "edited" signal, my callack will be called only when the
spin button is "unfocused". I lost every clicks.