Feature request: add a "Show read notifications" setting

[Arthur]

📝 Provide a description of the new feature

Thanks to the (coming 🤞) new Mark notification as done button (see #500 & #706), I think we could add a new "Show read notifications" setting, to better mimic the default Github's notifications experience.

The workflow would be the following:

With the setting ON:

1. You get a new notification, it appears in Gitify
2. You click on this notification, it opens and Github and automatically gets marked as "read"
3. It still appears inside Gitify, with a grey background, and you have the ability to click the new Mark as done button to make it disappear

With the setting OFF:

1. You get a new notification, it appears in Gitify
2. You click on this notification, it opens and Github and automatically gets marked as "read"
3. It doesn't appear inside Gitify anymore

➕ Additional Information

I'm willing to work on this if you think it could be a new feature!