[feature request] Enable app to overwrite existing backup file

[LakmirTT]

For example if smb uses syncthing to sync folder with .ics files, that would be cumbersome to adjust calendar on other machine because app creates new backup file during each backup interval.

[inson1]

@LakmirTT SMT apps were sold to ad company. Fork is here https://github.com/FossifyOrg