Add control for showing github notifications

[evroon]

Thanks for creating this nice repository!
This PR adds a control that shows the amount of notifications for a Github user.
It refreshes every minute by default and when the control is pressed, it opens the url of the first notification.
Only a github token is required in settings.yml, but the color of the text, browser and refresh rate can be modified as well.
If you want, I can update documentation etc.

[jamesridgway]

Hi @evroon I was planning to take a look at this and get this merged in? Do you still have interest in something like this?

[evroon]

Sure, thanks

[jamesridgway]

I'm happy with the GithubNotificationsControl itself, I was just wondering what the need is with the various nix files? It seems like an unrelated change?

[evroon]

Ah oops I forgot to put them on a separate branch, will remove that tomorrow

[evroon]

Sorry for the delay, I removed the nix commit now using a rebase.