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);
    }
@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