Improve handlings of files conflict between delete and update

[mgallien]

In case one is locally modifying a file that is shared (or in a group folder), the file can also be deleted.
You would then have a conflict between the delete from server and the file update from client.
Currently, the user is not being involved and we will always upload the local modifications.
Improve handling of this by reusing the current conflict handling infrastructure.
Offer the user the ability to be involved and decide what to do.
Beside involving the user, make it clear that any parent folders needed to upload the file will be created automatically. That may not be what the user wants.
A final constraint is that it should be ensured that the local modifications are preserved until the user as solved the conflict.