As far as I know about OS X, it seems pretty much all applications use
growl for all notifications so, that would be good for mac. I am not
really sure about windows though, as snarl is not present by default,
I think the best option would be to use snarl if it is present,
otherwise just show gtk message dialog.
BTW I just took a quick look at snarl's api, it doesn't seem you can
add buttons to it (like accept and decline), and as it is inspired by
growl, it may not allow that either. So, unless you find a way around
it, I think I will just be happy with gtk message dialog.
- Aqeel