Group delete notifications

[Ivansss]

At the moment one notification is sent for every deleted notification.
This is okay for normal NC notifications, but for Talk notifications, this might be improved.

Imagine the following case:
You have 100 unread messages/notifications in a chat.
Once you enter that chat, NC will send 100 delete push notifications to your mobile device (even if you have dismissed manually that notifications in your device).

Maybe we can group somehow that delete notifications? Like for example sending an array of notificationIds in just one delete notification.