Implement "lazy loading" of the directory tree

[Nicklas Nordborg]

On a server with lots of user's the file manager is almost impossible to use since the generation of the directory tree takes too long time (if it works at all). The directory tree should be lazily loaded, with only the top-level items shown to begin with. Subdirectories are loaded on demand as the user click on them.

This ticket depends on #1245.