Tray icon should indicate new unread messages

[tomhunter-gh]

The tray icon should somehow indicate if there are new unread messages. The icon would revert as soon as the main window is displayed even if not every new message is marked as 'read'.

[tgmayfield]

Using Vercue for comparison, it's fine for the tray monitor to always show current status, rather than changing when the main window is active.