messageTray: Summarize notifications when messages queue up

It is really annoying for the user to acknowledge multiple notifications
when they queue up. So, to prevent a notification flood that has to be
handled by the user one-by-one, a summarized-notification feature is
added which leaves a single summarized-notification for the user,
replacing multiple notifications if the number exceeds 1, which they may
or may not acknowledge. When this summarized-notification is acknowledged,
the message-tray is opened where they can view the notifications that were
summarized. This helps the user concentrate on his primary task
simultaneously informing them about the new notifications.

https://bugzilla.gnome.org/show_bug.cgi?id=702460