We should either be showing the message tray in the notification or the summary mode.
This is best achieved if the message tray actor contains both, and either one is shown
at any given time.
Queue notifications so that each queued notification is shown when the previous one
times out or when the user is done interacting with the message tray and moves
the mouse away from it. (In the future, we will have some sort of an indication that
there are queued notifications and a way to have the next notification displayed
without moving the mouse away from the message tray.)
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