I want to enhance my browser-based web application with functionality that enables management of local files and folders. E.g. folder tree structures should be synchronized
Your best bet might be to write a custom application that interacts with your web application. For example, Dropbox lets you synchronize files across computers by use of a background application that watches a Dropbox-enabled folder. It also lets you view your Dropbox files online through a web browser. The Dropbox web application then allows you to delete/move/copy files which is echoed in your local filesystem.