Show number of unread tweets

[IBBoard]

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Originally posted as baedert/corebird#106 by @ryanlerch: "When a user has "scroll to top on new tweets" disabled, it would be awesome if we could also tell the user how many tweets are remaining in the scrollback. The blue circle icon that appears on the toolbar icon tells the user if they have unread tweets, but not how many are remaining."

Corebird used to do something like this with a horizontal bar as per the mock-up at https://camo.githubusercontent.com/ea60bedf6ef33afd383f14beb1a2f10ec04c13e3/68747470733a2f2f662e636c6f75642e6769746875622e636f6d2f6173736574732f3539323235392f323130333335382f34396630363934612d386636622d313165332d386634372d3663356635366533623737392e676966 (but might not show a count, just a "load newer"), or we can more easily do it with the notification and show "99+" to limit the size when Cawbird is left open.

We would need to do the same for replies and DMs as well, for consistency.

[kumarshantanu]

If there was a choice of using Text title instead of icons for the sections, the title could have been used to accommodate the unread count, e.g. Home (236), Mentions (2), Messages (7) etc. As an end user, I value the unread count more than the aesthetics of icons over text title.