code: function getElementsVisibility(elements) {
  
    var result = {};
  
    elements.forEach(function(e) {
      result[e.id] = e.hidden;
    });
  
    return result;
  }
docs: helpers ////////////////////// 
  Return a map { elementId -> hiddenState}.
  
  @param {Array<djs.model.Shape>} elements
  
  @return {Object}