Implement Notification Center

Related to #166 (closed) but for the main screen. When a notification appears it only lasts a few seconds and then disappears. It would be nice if there were a mechanism like there is on regular Gnome Shell to review and dismiss notifications that appear on the screen and then disappear.

Presumably this would be something that would be accessible from the top bar and again, similar to how it's handled in Gnome Shell with a number to the left or right of the current time for the current unviewed notifications. Selecting that in some way would allow the user to view and dismiss the notifications.