Describe the request

The Mastodon notifications web UI makes a helpful distinction, with sub-tabs, between "All" and "Mentions", so that I can find only things I was recently mentioned in and forgot to reply to.

In comparison, Tuba puts the mentions amidst the torrent of likes and boosts in the "Notifications" tab, and the "Conversations" tab is empty (because I don't do DMs)... so one is a mess, the other is empty. Needs a better balance there ;)

Implementation Details

• This should be an option in settings.
• This should be only available to some fediverse backends. (Include which ones on the above field).
• This is client-only (and shouldn't sync with the instance).
• This follows the GNOME HIG.