support for replacing an existing notification (take 2)

This is a follow-up to !10 (closed) . As suggested by @peterhoeg1 I created a new MR with the requested changes:

• patch by Paul Collins has been attributed as such ( 16c781b6 )
• added man page items ( d056d421 )

First timer, so let me know if something still needs fixing. CI builds seem broken atm, created !16 (merged) for that.