Implement tray icon tooltip

[gkatev]

At the very least, have the tooltip show the number of currently unread emails. Might also be nice if we could show the subject and a preview of the email's body, if only one is unread, but it's probably not really possible using the folder.unread-updated event. Alternatively, we can show which folders have unread emails, and how many.

[gkatev]

Another idea, not strictly related, but makes more sense in conjunction with the tooltip: 'Dismiss' action in icon's right-click menu, or even inside the tooltip if possible, to set all currently unread mails as read, as far as the icon is concerned (not actually marking the emails themselves as read).