Factor UserAgent to fs and move Version to fs
drive: check errors in Open() better
rclonetest: check sub directory and downloads
dropbox: graphics for the Dropbox app (used in auth process)
dropbox: Initial support of full Fs interface

Still missing metadata support (eg SetModTime)
dropbox: basics of metadata in Dropbox datastore working
dropbox: Fix concurrent access to Dropbox datastore and Lower case keys in datastore
dropbox: Use /delta to list objects - much quicker

Also fix major performance problem - re-reading entry each time!
dropbox: open the datastore in the background
dropbox: Fix mkdir on already created directory
dropbox: remove metadata when we remove files
dropbox: make limited fs work (copy single file)
dropbox: Switch to using RFC3339 for time metadata