Add support of update and delete of push notifications

[marcel-space]

When sending a message with "extras: {tag: "example"}" you can update the push notification by sending the same tag. When sending a message with "extras: {tag: "example", cancel: "true"}" the push will be deleted. In the app all messages are shown.

When cancel the last push, a push with "New Messages" appears. I don't know where it come from.