Ask for confirmation to overwrite existing backups

[mvglasow]

As I just accidentally hit Backup instead of Restore (overwriting a good backup with a bad one): it would really be helpful to have a confirmation when overwriting an existing backup. Should be an easy one: just check if the target folder already exists and prompt if it does.

[nicoursi]

It happened to me and few times too by replacing an existing backup with an empty one 😃