code: function _addExtension(category, callback) {
          if (!callback || typeof callback !== "function") {
              console.error("Attempt to add FileTreeView", category, "extension without a callback function");
              return;
          }
          var callbackList = _extensions.get(category);
          if (!callbackList) {
              callbackList = Immutable.List();
          }
          callbackList = callbackList.push(callback);
          _extensions = _extensions.set(category, callbackList);
      }
docs: @private
  
  Add an extension for the given category (icons, addClass).
  
  @param {string} category Category to which the extension is being added
  @param {function} callback The extension function itself