notificate the user whenever new messages occure

[xoir]

It would be nice to use the systems notification for new messages. Preferably by enabling the user the switch between different styles of notifications (pulsing LED, vibration, sound, …) for new chat & group messages.

@PrasannaVenkadesh https://github.com/Marlinski/Rumble/pull/23 provides some code for vibration whenever new chat messages appear. This could only be a first step. I think we will need more and finer adjustable notifications in the future.