Only show new notifications in the screen shield

Rework the count system in Source, to distinguish between the
normal notification count and the count of unseen/unacknowledged
notification. (A notification is considered unacknowledged until
shown, as a banner or inside the summary box pointer).
Includes some code cleanups and a test for multiple notifications
in the same source.

https://bugzilla.gnome.org/show_bug.cgi?id=681143