Command stack in viewer

[nikku]

@tim-kilian

The commandStack is analog to the eventBus and should be in the core, so its available for everyone.

It is already available for everyone, it is simply not provided out of the box with every viewer.

To be clear:

• it has zero dependencies to modeling

• it can be included as easy as

  var viewer = new BpmnViewer({
    additionalModules: [
      commandModule
    ]
  });