Drive edit: Notify user when a document has been changed since last save

When using Direct Edit, notify user when file has been changed server-side since last save. This could be based on the initial file hash, or a change token (although change token is changed even if it is an other property than the file that has been modified). Present him a link to open the server-side file and decide what to do (keep server-side and close current or override server-side file)