Notification pane should reflect unread notifications

[eternaleye]

The notifications pane isn't very discoverable; if it got a red badge counting unread notifications, that could help.

[jfrederickson]

Seems reasonable - it's in the favicon/tray icon, but this would increase visibility of the notification pane.

It might also be useful to give those icons popovers like the icons in the bottom left? That'd also help with discoverability.

[grahamperrin]

Whilst there are problems (elsewhere) with mis-counts or apparent miscounts, I suggest a simple red badge that:

• does not count
• draws attention without a number.

[lampholder]

Related to my comment on #5277 - all those top right hand tabs are pretty hard to discover :\

I (think I) like the idea of drawing attention with a flag of some kind - would be good to see some design mockups.