Updated Callback and JavascriptFunction API

* Callbacks have now directly a JavascriptFunction instead of a String
which is translated later as a JavascriptFunction
* Callbacks have now a new constructor, allowing arguments to be added
internally
* Members renamed to be clearer
* The ExportManager now does not generate unused code any longer
* Unit tests updated accordingly
Fixed ExportLinkPosition related unit tests

And removed ugly System.out :-)