[WIP] Fix update detection for Dropbox

:no_entry: NO MERGE

Now using Dropbox hash as etag.
The current solution is clumsy and unefficient.

I think I'll have a try using the "rev" attribute next, as the current solution requires loading the whole directory listing, else Dropbox doesn't deliver the hash attr.
For some reason, when I read the hash from the storage cache it already has the latest value, so hasUpdated() is always false for dirs. Still, refreshing seems to work...

FYI @icewind1991