LPS-37685 Use up-to-date API to show chat desktop notifications

Hi Nate,

I didn't know really whom to send this to, so please feel free to redirect me to someone else if you don't have time to deal with it. Additionally, I've not opened a Story-type ticket before, so I'm not sure if I'm following the correct procedure.

I've been talking about this change in a forum thread a while back. Now that browsers started to support the W3C-defined API for notifications, we should probably step ahead move away from the webkit-prefixed API.
I believe we don't need to keep any kind of fallback to the webkit-specific one either, because it's only needed up to Chrome 19 (while the current Chrome version is 28) but please let me know if you have a different opinion there.

I've tested this on Chrome 20, 23, 28 and Firefox 22. I couldn't test it on Safari 6 though because you need at least OS X 10.8 for that feature.

Thanks!
Daniel