Allow custom FacesMessage Severities

[commodis]

There was once an issue regarding the Severity SUCCESS (#895 or javaee/javaserverfaces-spec#895) which was just closed for no documented reason.

Even though the class jakarta.faces.application.FacesMessage$Severity is public,
its constructor is private which disallows an instantiation.

[arjantijms]

Something in scope of 4.0 for sure. It was likely accidentally closed, as there were so many open issues then.

[thatsIch]

I guess it did not make the cut for 4.0 😄

[BalusC]

That'll be 5.0 then. This is indeed not an uncommon community demand.

[tandraschko]

@BalusC @arjantijms i think we can either do this or #1934

[commodis]

An enum would not be extensible.
If this is implemented as an enum then the only business requirement I have is to add the SUCCESS.
It seems MyFaces has already implemented it (#1592).

This is also seen in PrimeNG that this is commonly available (https://www.primefaces.org/primeng-v8-lts/#/messages).

If Mojarra also implements SUCCESS this issue here can be retracted.

[tandraschko]

then i would prefer a ENUM over this ticket as we already have SUCCESS and this was the only requested severity since years.