Improve Test API Code Generator for usage in Java

[tschlat]

User story
The Citrus Test API code generator (#1156) has been developed with focus on XML. Usage in Java breaks the fluent API provided by Citrus,
as it currently does not support the Builder pattern used throught Citrus Java DLS.

The Builder Pattern approach has to be added into respective templates.

When finished, check for all occurrences of TODO #1161 in Citrus code base..