I saw something similar on scrutinizer and I found it quite nice, so I decided to give it a go.
It's still in a very early WIP, so basically two things exist:
1) the code to grab the unread PMs,
2) the UI (or most of it).
If you have unread messages an envelop is shown in the bottom-right corner of the window (attachment 1 - screen_299), clicking on it will load 5 unread messages you have (attachment 1 - screen_300) and there you can control the basics of PMs: reply to all (attachment 3 - screen_301) or delete.
I tried to use FA as much as possible, I'm not sure if the choice of icons is very good, but should be understandable... I hope.
The aspect still lacks a bit of whistles, but I'm not that good in this kind of stuff, so any suggestion is welcome. ;D
ETA:
Created the repository at https://github.com/emanuele45/PmNotification
Thats pretty cool 8)
I'm working now on the controller to send the new PM, and I have to say that the PM controller is still a pain! (And a mess LOL)
Pushed a working reply! :D
ETA: new screen with the confirmation the PM has been sent.
Nice...
You know, I'm almost tempted to propose it for 1.1 instead of the popup thingy... O:-)