code: function (category, icon) {
                      if (typeof icon === 'undefined') {
                          return icons[category];
                      }
  
                      var previous = icons[category];
                      icons[category] = icon;
                      return previous;
                  }
docs: @ngdoc function
  @name w20CoreEnv.service:PreferencesService#icon
  @methodOf w20CoreEnv.service:PreferencesService
  @param {String} category The preference category
  @param {icon} icon The type of the icon (i.e. the CSS class of the icon) to set.
  @returns {String} The type of the icon if used as a getter, the previous type of icon if used as a setter.
  
  @description
  
  Getter/setter for the icon of a preference category.