code: function addonsManager_categories(aSpec) {
      var spec = aSpec || { };
  
      var categories = this.getElements({
        type: "categories",
        subtype: spec.attribute,
        value: spec.value
      });
  
      if (categories.length == 0)
        throw new Error(arguments.callee.name + ": Categories could not be found.");
  
      return categories;
    }
docs: Returns the categories which match the filter criteria
  
  @param {object} aSpec
  Information about the filter to apply
  Elements: attribute - DOM attribute of the wanted category
  [optional - default: ""]
  value     - Value of the DOM attribute
  [optional - default: ""]
  
  @returns List of categories
  @type {array of ElemBase}