Replace @Optional with @OptArg or similar

[DarkArc]

Currently if using the @Optional annotation to specify an optional argument, within a class file also using the JDK Optional class, or the Guava Optional class you're forced to use the expanded type name so as to specify which Optional you're referring to.

To resolve this naming conflict, I suggest making the @Optional annotation an @OptArg annotation or similar.

[elysweyr]

What about this issure? It's still open: what'll happen?