Feature request: add a "Show read notifications" setting

[adufr]

📝 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!

[kacperpONS]

I second this!
It's really annoying, because currently once you click the notification in Gitlify it disappears from there permanently and there's no way to go back to it. It would be incredibly useful to add a filtering option to show all the notifications.

It would also be convenient to add a Mark as read button that would turn to Mark as unread for the read notifications.

[setchy]

I second this! It's really annoying, because currently once you click the notification in Gitlify it disappears from there permanently and there's no way to go back to it.

Time to dust off #1052 ;)

It would be incredibly useful to add a filtering option to show all the notifications.

I like the suggestion of moving it to be a "filter" type (I've done similar with atlassify.io actually)

It would also be convenient to add a Mark as read button that would turn to Mark as unread for the read notifications.

Unfortunately, as best as I could tell, there isn't any public API for mark as unread - #890