_getElementName (element) {
    return Object.keys(this.dom)
      .find(name => this.dom[name] === element)
  }
Get the dom name of given element. returns null if not found.
For example when element === dom.field, "field" is returned.
@param {Element} element
@return {String | null} elementName  Available elements with name: 'drag',
                                     'menu', 'expand', 'field', 'value'
@private