Notifications should stay hidden once they are dismissed

Given you get a notification about `A` and you swipe it away to dismiss it, and a new notification `B` arrives, i'd expect **only** `B` to be shown.
Currently it shows `A` **and** `B` which is nagging because the notification recurs apparently on each synchronization.

Attention: The "Mark as read" action of the notification must only be applied to the notifications that are shown, so given you dismissed `A`, it should only not appear in the notifications again - the state is still `unread`. Given `B` arrives, and we trigger the "Mark as read" action in the notifications, only `B` must be set to `read`.

cc @desperateCoder

Update: The wording of the notification action should of course be `Mark as read` instead of `Mark all as read`.