javafx.scene.input.*Event classes construction

[Richard Bair]

>> As for the approach, I think you do the constructors with all params (since events are immutable you have no choice really -- static factory or constructor and I prefer in this case a constructor) + builder (auto generated).
> And what do you think about impl_copy methods? Personally I think we should remove them completely and turn them to some internal utility methods.

My initial thought was a copy constructor.

> Also, some events are not 100% immutable, as they are modified during the Scene processing through some impl_methods, like MouseEvent.impl_setClickParam. We'd either need to make some/all of the Event fields protected and do this modifications through subclasses or create some "accessor" in javafx.scene.input package for calling these methods from javafx.scene package.

Good question, I guess we can revisit these in context of the other impl_ method removal things we're working on.

Richard