Add a barebone DropboxSource implementation
Add extension method to call Fetch on destination instance, e.g. _myFolder.Fetch ('dropbox://myfile.png')
Add helpers for saving in Library and Library/Caches
Rewrite this to be shorter & consistent
No need to Select--just use non-generic Task overload.
Prevent sources, and their stuff, from getting garbage-collected. This fixes Dropbox.