(Re-)implement getItemURL for Dropbox

[galfert]

Current (inactive) implementation:

To get the public URL for a shared file in Dropbox, the client needs to make a request to Dropbox to create/get that URL. Since that is asynchronous, the Dropbox client keeps a local cache (_itemRefs) of those URLs which is then used by getItemURL.

But the cache is only filled when either GETing or PUTing a file. So when a directory is cached, the URLs are only available after the first sync cycle. But when caching is disabled, getItemURL always returns and empty string.

[galfert]

Also, for private files, getItemURL just returns an empty string as well.