Display a warning when a user starts a concurrent editing

The request is to display a warning in the user's environment where she begins to edit a local file, synchronized with Drive, whereas another user is already editing the document.

The warning would prevent from having a conflict where the 2 users will save their changes and Drive will display an error during the synchronization

Steps to reproduce:

1. pre-requisites: 2 different users have synchronized the same folder
2. both users use their local file explorer to navigate to the folder synchronized with Drive
3. they both open the same file locally, with MS Word for example
4. at that step, the Nuxeo server locks the document for one of the users
5. both users edit and save the document
6. Nuxeo Drive throws a synchronization error for one of the user, the one which did not get the lock on the document

Expected behavior: at step 4, or some seconds after the lock is done, the user which opened the file receives an alert that the document has been locked by another user.

Note that the file are not edited with Direct-Edit in this scenario