Refactor:  move the pollTimer to the folder
Fetch the etag of the folder to see if it has changed before syncing

Notice that this does not work for the toplevel directory yet, as
owncloud does not change the etag of the root folder
Get the etag of all subfolder for the root directory

The root directory etag is not updated when the files change.
That's why we need to check the etag of all its sub items