Fixes#1393
The objects returned by `new Notification()` have a `.close()` method.
So we can just keep a list of notifications that have been sent, and
close them when that message has been read.
Conflicts:
internal_packages/unread-notifications/lib/main.es6
* Remove double negative from test label
It always takes me a second to read a double negative, and be sure I
understood correctly. It's also particularly troublesome for non-native
English speakers.
* Change pending unread notification stack to be a queue
Consider the case where a user launches N1, and has 3 unreads since
their last launch. Additionally, after N1 has launched, they begin
receiving a new mail every 2 seconds.
Current behavior would be a notification for 1 of the 3 unread emails,
then every 2 seconds a notification for the most recently received mail.
They would only finally receive notifications for the other 2 original
messages once they stopped getting new mail.
