avoid usage of QScriptContext::callee in callbacks

In order to fix QBS-695 we must work-around QTBUG-39416.
Instead of relying on the callee() and extracting data from it, we're
passing payload via QScriptEngine::newFunction.

Change-Id: I1d454fba78523c7cd77a906952bdd48cbd5330db
Task-number: QBS-695
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>