Copy element to other model

Making it work within the same browser window is supported out of the box, cf. bpmn-js#97e1ad14.

The trick is to share a clipboard module across multiple bpmn-js instances like so:

var clipboardModule = {
  'clipboard': [ 'value', new Clipboard() ]
};

m2 = new Modeler({
  additionalModules: [
    clipboardModule
  ]
});

m1 = new Modeler({
  additionalModules: [
    clipboardModule
  ]
});