"Mark all as read" should only apply to selected notification type

[obigriffith]

I think it would be more intuitive if the "Mark all as read" button only applied to all notifications with in the selected tab/category. I.e., if on the "All" tab then apply to all. If on the "Subscribed events" tab, apply only to all subscribed events. If on the "mentions" tab, apply only to all mentions. And so on. I had the case where I looked through all my mentions and decided I was done with them. I wanted to mark all as read but unintentionally marked all my other notifications. :-(