[Stunner] PoC - Achieve end to end marhalling support

The POC provided as a result of the work in JBPM-8586 is load-only. Besides making the code compile client-side, no further steps were taken to verify that an XML DOM is actually generated.

Goal of this task is to verify that the "to XML" marshalling process produces meaningful files.
e.g. namespace-specific APIs such as createElementNS are not supported by the GWT XML DOM shim, so were substitute by createElement. All such changes should be marked with a fixme comment so they should be easy to track down.

Although not strictly necessary, it seems simpler to first adapt the standalone demo app found here https://github.com/evacchi/gwt-bpmn-marshallers to *generate* XML using the ECore API instead of just *loading* it. In practice, this would mean to use the Resource API to get a XML DOM object and then print its toString() (the XML string).

Second step would be to actually connect the pieces together, by porting the backend code to the client-side in the stunner standalone showcase.