[pygtk] using a generic getter for all gints

[John Ehresman]

Rafael Espíndola wrote:
> The idea of using a single getter for all gint attributes was simpler
> to implement than expected. The API provides a closure pointer that
> makes it very easy to pass the offset.

Patches like this need to be in bugzilla.

Something like this is probably a step forward, though details such as 
whether it's portable to other compilers, whether pyg_boxed_get is 
needed (I think you're just casting to char *), and if an extra header 
file is needed should be worked out.

John