**events example in oberon**

Just a simple example which helps to explain how events are implemented, what is callback procedure, and how easy it is.

Some languages simplify programmer's work, hide details, so that it's not obvious to him what's going on when he defines it's own OnSomething function.

Callbacks are common when dealing with gui events like OnClick but they can be used with any events. I guess code is simple enough, it doesn't need explanation.
