A workaround for multiple repeating notifications

[laughedelic]

On every new notification it goes through all matching old notifications
and dismisses them. We can do it only after the fact. Also we cannot
dismiss the new one instead, because if some old ones were dismissed
manually, user won't see the new notifications.

Fixes #20.

(in this demo server sends warnings on any action, normally it shouldn't send them so often)

[laughedelic]

Notice that this is a general problem in the atom-notifications package and should be solved there. See atom/notifications#88 and other issues linked there. This is merely a hacky workaround.

I'm quite limited here, because I can only do things after notification has been displayed:

• I can't easily hook to the server showMessage request to do something before it is received by the client
• There is no onWillAddNotification hook in the Atom NotificationManager API, only onDidAddNotification

Yet, @cquiroz if you have any better idea how to handle it, let me know.

[laughedelic]

The particular message which caused this issue should be gone after scalameta/metals#116, but I'm going to merge this anyway for any other potentially annoying warnings.