Adding named slot for system messages

[lterlueuen]

It would be great if there would be named slots for system messages. This is needed in our case for example to
attach a timestamp or a link to the system message.

[antoine92190]

I agree, I can add that soon, or you can create a PR if you want this quicker.

[lterlueuen]

It would be nice if you could add that.

[antoine92190]

Added in c31a87c